diff --git a/analytics/lib/counts.py b/analytics/lib/counts.py
index 87a51e5fa6..ce8ab4ce14 100644
--- a/analytics/lib/counts.py
+++ b/analytics/lib/counts.py
@@ -40,7 +40,7 @@ class CountStat:
         self.data_collector = data_collector
         # might have to do something different for bitfields
         if frequency not in self.FREQUENCIES:
-            raise AssertionError("Unknown frequency: %s" % (frequency,))
+            raise AssertionError(f"Unknown frequency: {frequency}")
         self.frequency = frequency
         if interval is not None:
             self.interval = interval
@@ -50,7 +50,7 @@ class CountStat:
             self.interval = timedelta(days=1)
 
     def __str__(self) -> str:
-        return "<CountStat: %s>" % (self.property,)
+        return f"<CountStat: {self.property}>"
 
 class LoggingCountStat(CountStat):
     def __init__(self, property: str, output_table: Type[BaseCount], frequency: str) -> None:
@@ -86,11 +86,11 @@ def process_count_stat(stat: CountStat, fill_to_time: datetime,
     elif stat.frequency == CountStat.DAY:
         time_increment = timedelta(days=1)
     else:
-        raise AssertionError("Unknown frequency: %s" % (stat.frequency,))
+        raise AssertionError(f"Unknown frequency: {stat.frequency}")
 
     verify_UTC(fill_to_time)
     if floor_to_hour(fill_to_time) != fill_to_time:
-        raise ValueError("fill_to_time must be on an hour boundary: %s" % (fill_to_time,))
+        raise ValueError(f"fill_to_time must be on an hour boundary: {fill_to_time}")
 
     fill_state = FillState.objects.filter(property=stat.property).first()
     if fill_state is None:
@@ -108,7 +108,7 @@ def process_count_stat(stat: CountStat, fill_to_time: datetime,
     elif fill_state.state == FillState.DONE:
         currently_filled = fill_state.end_time
     else:
-        raise AssertionError("Unknown value for FillState.state: %s." % (fill_state.state,))
+        raise AssertionError(f"Unknown value for FillState.state: {fill_state.state}.")
 
     if isinstance(stat, DependentCountStat):
         for dependency in stat.dependencies:
diff --git a/analytics/lib/fixtures.py b/analytics/lib/fixtures.py
index bd24940416..a439b56d95 100644
--- a/analytics/lib/fixtures.py
+++ b/analytics/lib/fixtures.py
@@ -43,7 +43,7 @@ def generate_time_series_data(days: int=100, business_hours_base: float=10,
                       [24*non_business_hours_base] * 2
         holidays = [random() < holiday_rate for i in range(days)]
     else:
-        raise AssertionError("Unknown frequency: %s" % (frequency,))
+        raise AssertionError(f"Unknown frequency: {frequency}")
     if length < 2:
         raise AssertionError("Must be generating at least 2 data points. "
                              "Currently generating %s" % (length,))
diff --git a/analytics/lib/time_utils.py b/analytics/lib/time_utils.py
index e372125430..48d8f54223 100644
--- a/analytics/lib/time_utils.py
+++ b/analytics/lib/time_utils.py
@@ -19,7 +19,7 @@ def time_range(start: datetime, end: datetime, frequency: str,
         end = floor_to_day(end)
         step = timedelta(days=1)
     else:
-        raise AssertionError("Unknown frequency: %s" % (frequency,))
+        raise AssertionError(f"Unknown frequency: {frequency}")
 
     times = []
     if min_length is not None:
diff --git a/analytics/management/commands/analyze_user_activity.py b/analytics/management/commands/analyze_user_activity.py
index 0e41d6deaa..19322082e2 100644
--- a/analytics/management/commands/analyze_user_activity.py
+++ b/analytics/management/commands/analyze_user_activity.py
@@ -26,9 +26,9 @@ def analyze_activity(options: Dict[str, Any]) -> None:
         total_duration += duration
         print("%-*s%s" % (37, user_profile.email, duration,))
 
-    print("\nTotal Duration:                      %s" % (total_duration,))
-    print("\nTotal Duration in minutes:           %s" % (total_duration.total_seconds() / 60.,))
-    print("Total Duration amortized to a month: %s" % (total_duration.total_seconds() * 30. / 60.,))
+    print(f"\nTotal Duration:                      {total_duration}")
+    print(f"\nTotal Duration in minutes:           {total_duration.total_seconds() / 60.}")
+    print(f"Total Duration amortized to a month: {total_duration.total_seconds() * 30. / 60.}")
 
 class Command(BaseCommand):
     help = """Report analytics of user activity on a per-user and realm basis.
diff --git a/analytics/management/commands/check_analytics_state.py b/analytics/management/commands/check_analytics_state.py
index 8e7d903163..464659b68f 100644
--- a/analytics/management/commands/check_analytics_state.py
+++ b/analytics/management/commands/check_analytics_state.py
@@ -33,8 +33,7 @@ class Command(BaseCommand):
         state_file_tmp = state_file_path + "-tmp"
 
         with open(state_file_tmp, "w") as f:
-            f.write("%s|%s|%s|%s\n" % (
-                int(time.time()), status, states[status], message))
+            f.write(f"{int(time.time())}|{status}|{states[status]}|{message}\n")
         os.rename(state_file_tmp, state_file_path)
 
     def get_fill_state(self) -> Dict[str, Any]:
@@ -50,7 +49,7 @@ class Command(BaseCommand):
             try:
                 verify_UTC(last_fill)
             except TimezoneNotUTCException:
-                return {'status': 2, 'message': 'FillState not in UTC for %s' % (property,)}
+                return {'status': 2, 'message': f'FillState not in UTC for {property}'}
 
             if stat.frequency == CountStat.DAY:
                 floor_function = floor_to_day
diff --git a/analytics/management/commands/clear_single_stat.py b/analytics/management/commands/clear_single_stat.py
index cc004bc444..ff5964ddd3 100644
--- a/analytics/management/commands/clear_single_stat.py
+++ b/analytics/management/commands/clear_single_stat.py
@@ -20,7 +20,7 @@ class Command(BaseCommand):
     def handle(self, *args: Any, **options: Any) -> None:
         property = options['property']
         if property not in COUNT_STATS:
-            raise CommandError("Invalid property: %s" % (property,))
+            raise CommandError(f"Invalid property: {property}")
         if not options['force']:
             raise CommandError("No action taken. Use --force.")
 
diff --git a/analytics/management/commands/realm_stats.py b/analytics/management/commands/realm_stats.py
index fb8cf2a0cf..8730a3a217 100644
--- a/analytics/management/commands/realm_stats.py
+++ b/analytics/management/commands/realm_stats.py
@@ -66,7 +66,7 @@ class Command(BaseCommand):
             fraction = 0.0
         else:
             fraction = numerator / float(denominator)
-        print("%.2f%% of" % (fraction * 100,), text)
+        print(f"{fraction * 100:.2f}% of", text)
 
     def handle(self, *args: Any, **options: Any) -> None:
         if options['realms']:
diff --git a/analytics/management/commands/stream_stats.py b/analytics/management/commands/stream_stats.py
index d96efbbcb7..b6bde4bdc2 100644
--- a/analytics/management/commands/stream_stats.py
+++ b/analytics/management/commands/stream_stats.py
@@ -40,18 +40,18 @@ class Command(BaseCommand):
             print("%10s %d public streams and" % ("(", public_count), end=' ')
             print("%d private streams )" % (private_count,))
             print("------------")
-            print("%25s %15s %10s %12s" % ("stream", "subscribers", "messages", "type"))
+            print("{:>25} {:>15} {:>10} {:>12}".format("stream", "subscribers", "messages", "type"))
 
             for stream in streams:
                 if stream.invite_only:
                     stream_type = 'private'
                 else:
                     stream_type = 'public'
-                print("%25s" % (stream.name,), end=' ')
+                print(f"{stream.name:>25}", end=' ')
                 recipient = Recipient.objects.filter(type=Recipient.STREAM, type_id=stream.id)
                 print("%10d" % (len(Subscription.objects.filter(recipient=recipient,
                                                                 active=True)),), end=' ')
                 num_messages = len(Message.objects.filter(recipient=recipient))
                 print("%12d" % (num_messages,), end=' ')
-                print("%15s" % (stream_type,))
+                print(f"{stream_type:>15}")
             print("")
diff --git a/analytics/management/commands/update_analytics_counts.py b/analytics/management/commands/update_analytics_counts.py
index 886b744853..9e89bb2246 100644
--- a/analytics/management/commands/update_analytics_counts.py
+++ b/analytics/management/commands/update_analytics_counts.py
@@ -71,7 +71,7 @@ class Command(BaseCommand):
         else:
             stats = list(COUNT_STATS.values())
 
-        logger.info("Starting updating analytics counts through %s" % (fill_to_time,))
+        logger.info(f"Starting updating analytics counts through {fill_to_time}")
         if options['verbose']:
             start = time.time()
             last = start
@@ -79,13 +79,13 @@ class Command(BaseCommand):
         for stat in stats:
             process_count_stat(stat, fill_to_time)
             if options['verbose']:
-                print("Updated %s in %.3fs" % (stat.property, time.time() - last))
+                print(f"Updated {stat.property} in {time.time() - last:.3f}s")
                 last = time.time()
 
         if options['verbose']:
             print("Finished updating analytics counts through %s in %.3fs" %
                   (fill_to_time, time.time() - start))
-        logger.info("Finished updating analytics counts through %s" % (fill_to_time,))
+        logger.info(f"Finished updating analytics counts through {fill_to_time}")
 
         if settings.PUSH_NOTIFICATION_BOUNCER_URL and settings.SUBMIT_USAGE_STATISTICS:
             send_analytics_to_remote_server()
diff --git a/analytics/management/commands/user_stats.py b/analytics/management/commands/user_stats.py
index 2b023db0e4..ae83b14cc8 100644
--- a/analytics/management/commands/user_stats.py
+++ b/analytics/management/commands/user_stats.py
@@ -36,7 +36,7 @@ class Command(BaseCommand):
             print("%d streams" % (len(Stream.objects.filter(realm=realm)),))
 
             for user_profile in user_profiles:
-                print("%35s" % (user_profile.email,), end=' ')
+                print(f"{user_profile.email:>35}", end=' ')
                 for week in range(10):
                     print("%5d" % (self.messages_sent_by(user_profile, week),), end=' ')
                 print("")
diff --git a/analytics/models.py b/analytics/models.py
index 4d6ebd11de..491e144ac0 100644
--- a/analytics/models.py
+++ b/analytics/models.py
@@ -17,7 +17,7 @@ class FillState(models.Model):
     state: int = models.PositiveSmallIntegerField()
 
     def __str__(self) -> str:
-        return "<FillState: %s %s %s>" % (self.property, self.end_time, self.state)
+        return f"<FillState: {self.property} {self.end_time} {self.state}>"
 
 # The earliest/starting end_time in FillState
 # We assume there is at least one realm
@@ -61,7 +61,7 @@ class InstallationCount(BaseCount):
         ]
 
     def __str__(self) -> str:
-        return "<InstallationCount: %s %s %s>" % (self.property, self.subgroup, self.value)
+        return f"<InstallationCount: {self.property} {self.subgroup} {self.value}>"
 
 class RealmCount(BaseCount):
     realm = models.ForeignKey(Realm, on_delete=models.CASCADE)
@@ -81,7 +81,7 @@ class RealmCount(BaseCount):
         index_together = ["property", "end_time"]
 
     def __str__(self) -> str:
-        return "<RealmCount: %s %s %s %s>" % (self.realm, self.property, self.subgroup, self.value)
+        return f"<RealmCount: {self.realm} {self.property} {self.subgroup} {self.value}>"
 
 class UserCount(BaseCount):
     user = models.ForeignKey(UserProfile, on_delete=models.CASCADE)
@@ -104,7 +104,7 @@ class UserCount(BaseCount):
         index_together = ["property", "realm", "end_time"]
 
     def __str__(self) -> str:
-        return "<UserCount: %s %s %s %s>" % (self.user, self.property, self.subgroup, self.value)
+        return f"<UserCount: {self.user} {self.property} {self.subgroup} {self.value}>"
 
 class StreamCount(BaseCount):
     stream = models.ForeignKey(Stream, on_delete=models.CASCADE)
@@ -127,5 +127,4 @@ class StreamCount(BaseCount):
         index_together = ["property", "realm", "end_time"]
 
     def __str__(self) -> str:
-        return "<StreamCount: %s %s %s %s %s>" % (
-            self.stream, self.property, self.subgroup, self.value, self.id)
+        return f"<StreamCount: {self.stream} {self.property} {self.subgroup} {self.value} {self.id}>"
diff --git a/analytics/tests/test_counts.py b/analytics/tests/test_counts.py
index dcc0aef4a1..7992da7af6 100644
--- a/analytics/tests/test_counts.py
+++ b/analytics/tests/test_counts.py
@@ -49,7 +49,7 @@ class AnalyticsTestCase(TestCase):
     def create_user(self, **kwargs: Any) -> UserProfile:
         self.name_counter += 1
         defaults = {
-            'email': 'user%s@domain.tld' % (self.name_counter,),
+            'email': f'user{self.name_counter}@domain.tld',
             'date_joined': self.TIME_LAST_HOUR,
             'full_name': 'full_name',
             'short_name': 'short_name',
@@ -71,7 +71,7 @@ class AnalyticsTestCase(TestCase):
 
     def create_stream_with_recipient(self, **kwargs: Any) -> Tuple[Stream, Recipient]:
         self.name_counter += 1
-        defaults = {'name': 'stream name %s' % (self.name_counter,),
+        defaults = {'name': f'stream name {self.name_counter}',
                     'realm': self.default_realm,
                     'date_created': self.TIME_LAST_HOUR}
         for key, value in defaults.items():
@@ -84,7 +84,7 @@ class AnalyticsTestCase(TestCase):
 
     def create_huddle_with_recipient(self, **kwargs: Any) -> Tuple[Huddle, Recipient]:
         self.name_counter += 1
-        defaults = {'huddle_hash': 'hash%s' % (self.name_counter,)}
+        defaults = {'huddle_hash': f'hash{self.name_counter}'}
         for key, value in defaults.items():
             kwargs[key] = kwargs.get(key, value)
         huddle = Huddle.objects.create(**kwargs)
@@ -339,10 +339,10 @@ class TestCountStats(AnalyticsTestCase):
             date_created=self.TIME_ZERO-2*self.DAY)
         for minutes_ago in [0, 1, 61, 60*24+1]:
             creation_time = self.TIME_ZERO - minutes_ago*self.MINUTE
-            user = self.create_user(email='user-%s@second.analytics' % (minutes_ago,),
+            user = self.create_user(email=f'user-{minutes_ago}@second.analytics',
                                     realm=self.second_realm, date_joined=creation_time)
             recipient = self.create_stream_with_recipient(
-                name='stream %s' % (minutes_ago,), realm=self.second_realm,
+                name=f'stream {minutes_ago}', realm=self.second_realm,
                 date_created=creation_time)[1]
             self.create_message(user, recipient, date_sent=creation_time)
         self.hourly_user = get_user('user-1@second.analytics', self.second_realm)
diff --git a/analytics/tests/test_views.py b/analytics/tests/test_views.py
index aec6e365d7..6e4de62be8 100644
--- a/analytics/tests/test_views.py
+++ b/analytics/tests/test_views.py
@@ -408,7 +408,7 @@ class TestSupportEndpoint(ZulipTestCase):
 
         def check_zulip_realm_query_result(result: HttpResponse) -> None:
             zulip_realm = get_realm("zulip")
-            self.assert_in_success_response(['<input type="hidden" name="realm_id" value="%s"' % (zulip_realm.id,),
+            self.assert_in_success_response([f'<input type="hidden" name="realm_id" value="{zulip_realm.id}"',
                                              'Zulip Dev</h3>',
                                              '<option value="1" selected>Self Hosted</option>',
                                              '<option value="2" >Limited</option>',
@@ -420,7 +420,7 @@ class TestSupportEndpoint(ZulipTestCase):
 
         def check_lear_realm_query_result(result: HttpResponse) -> None:
             lear_realm = get_realm("lear")
-            self.assert_in_success_response(['<input type="hidden" name="realm_id" value="%s"' % (lear_realm.id,),
+            self.assert_in_success_response([f'<input type="hidden" name="realm_id" value="{lear_realm.id}"',
                                              'Lear &amp; Co.</h3>',
                                              '<option value="1" selected>Self Hosted</option>',
                                              '<option value="2" >Limited</option>',
@@ -532,7 +532,7 @@ class TestSupportEndpoint(ZulipTestCase):
         cordelia = self.example_user('cordelia')
         self.login_user(cordelia)
 
-        result = self.client_post("/activity/support", {"realm_id": "%s" % (cordelia.realm_id,), "plan_type": "2"})
+        result = self.client_post("/activity/support", {"realm_id": f"{cordelia.realm_id}", "plan_type": "2"})
         self.assertEqual(result.status_code, 302)
         self.assertEqual(result["Location"], "/login/")
 
@@ -540,7 +540,7 @@ class TestSupportEndpoint(ZulipTestCase):
         self.login_user(iago)
 
         with mock.patch("analytics.views.do_change_plan_type") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (iago.realm_id,), "plan_type": "2"})
+            result = self.client_post("/activity/support", {"realm_id": f"{iago.realm_id}", "plan_type": "2"})
             m.assert_called_once_with(get_realm("zulip"), 2)
             self.assert_in_success_response(["Plan type of Zulip Dev changed from self hosted to limited"], result)
 
@@ -549,14 +549,14 @@ class TestSupportEndpoint(ZulipTestCase):
         lear_realm = get_realm('lear')
         self.login_user(cordelia)
 
-        result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "discount": "25"})
+        result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "discount": "25"})
         self.assertEqual(result.status_code, 302)
         self.assertEqual(result["Location"], "/login/")
 
         self.login('iago')
 
         with mock.patch("analytics.views.attach_discount_to_realm") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "discount": "25"})
+            result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "discount": "25"})
             m.assert_called_once_with(get_realm("lear"), 25)
             self.assert_in_success_response(["Discount of Lear &amp; Co. changed to 25 from None"], result)
 
@@ -565,19 +565,19 @@ class TestSupportEndpoint(ZulipTestCase):
         lear_realm = get_realm('lear')
         self.login_user(cordelia)
 
-        result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "status": "deactivated"})
+        result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "status": "deactivated"})
         self.assertEqual(result.status_code, 302)
         self.assertEqual(result["Location"], "/login/")
 
         self.login('iago')
 
         with mock.patch("analytics.views.do_deactivate_realm") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "status": "deactivated"})
+            result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "status": "deactivated"})
             m.assert_called_once_with(lear_realm, self.example_user("iago"))
             self.assert_in_success_response(["Lear &amp; Co. deactivated"], result)
 
         with mock.patch("analytics.views.do_send_realm_reactivation_email") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "status": "active"})
+            result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "status": "active"})
             m.assert_called_once_with(lear_realm)
             self.assert_in_success_response(["Realm reactivation email sent to admins of Lear"], result)
 
@@ -586,19 +586,19 @@ class TestSupportEndpoint(ZulipTestCase):
         lear_realm = get_realm('lear')
         self.login_user(cordelia)
 
-        result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "discount": "25"})
+        result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "discount": "25"})
         self.assertEqual(result.status_code, 302)
         self.assertEqual(result["Location"], "/login/")
 
         self.login('iago')
 
         with mock.patch("analytics.views.do_scrub_realm") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,), "scrub_realm": "scrub_realm"})
+            result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "scrub_realm": "scrub_realm"})
             m.assert_called_once_with(lear_realm)
             self.assert_in_success_response(["Lear &amp; Co. scrubbed"], result)
 
         with mock.patch("analytics.views.do_scrub_realm") as m:
-            result = self.client_post("/activity/support", {"realm_id": "%s" % (lear_realm.id,)})
+            result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}"})
             m.assert_not_called()
 
 class TestGetChartDataHelpers(ZulipTestCase):
diff --git a/analytics/views.py b/analytics/views.py
index 4987344709..d818a1d229 100644
--- a/analytics/views.py
+++ b/analytics/views.py
@@ -96,9 +96,9 @@ def stats_for_realm(request: HttpRequest, realm_str: str) -> HttpResponse:
     try:
         realm = get_realm(realm_str)
     except Realm.DoesNotExist:
-        return HttpResponseNotFound("Realm %s does not exist" % (realm_str,))
+        return HttpResponseNotFound(f"Realm {realm_str} does not exist")
 
-    return render_stats(request, '/realm/%s' % (realm_str,), realm.name or realm.string_id,
+    return render_stats(request, f'/realm/{realm_str}', realm.name or realm.string_id,
                         analytics_ready=is_analytics_ready(realm))
 
 @require_server_admin
@@ -106,8 +106,8 @@ def stats_for_realm(request: HttpRequest, realm_str: str) -> HttpResponse:
 def stats_for_remote_realm(request: HttpRequest, remote_server_id: str,
                            remote_realm_id: str) -> HttpResponse:
     server = RemoteZulipServer.objects.get(id=remote_server_id)
-    return render_stats(request, '/remote/%s/realm/%s' % (server.id, remote_realm_id),
-                        "Realm %s on server %s" % (remote_realm_id, server.hostname))
+    return render_stats(request, f'/remote/{server.id}/realm/{remote_realm_id}',
+                        f"Realm {remote_realm_id} on server {server.hostname}")
 
 @require_server_admin_api
 @has_request_variables
@@ -136,8 +136,8 @@ def stats_for_installation(request: HttpRequest) -> HttpResponse:
 @require_server_admin
 def stats_for_remote_installation(request: HttpRequest, remote_server_id: str) -> HttpResponse:
     server = RemoteZulipServer.objects.get(id=remote_server_id)
-    return render_stats(request, '/remote/%s/installation' % (server.id,),
-                        'remote Installation %s' % (server.hostname,), True, True)
+    return render_stats(request, f'/remote/{server.id}/installation',
+                        f'remote Installation {server.hostname}', True, True)
 
 @require_server_admin_api
 @has_request_variables
@@ -332,7 +332,7 @@ def table_filtered_to_id(table: Type[BaseCount], key_id: int) -> QuerySet:
     elif table == RemoteRealmCount:
         return RemoteRealmCount.objects.filter(realm_id=key_id)
     else:
-        raise AssertionError("Unknown table: %s" % (table,))
+        raise AssertionError(f"Unknown table: {table}")
 
 def client_label_map(name: str) -> str:
     if name == "website":
@@ -463,7 +463,7 @@ def get_realm_day_counts() -> Dict[str, Dict[str, str]]:
             else:
                 good_bad = 'neutral'
 
-            return '<td class="number %s">%s</td>' % (good_bad, cnt)
+            return f'<td class="number {good_bad}">{cnt}</td>'
 
         cnts = (format_count(raw_cnts[0], 'neutral')
                 + ''.join(map(format_count, raw_cnts[1:])))
@@ -629,7 +629,7 @@ def realm_summary_table(realm_minutes: Dict[str, float]) -> str:
         total_hours += hours
         row['hours'] = str(int(hours))
         try:
-            row['hours_per_user'] = '%.1f' % (hours / row['dau_count'],)
+            row['hours_per_user'] = '{:.1f}'.format(hours / row['dau_count'])
         except Exception:
             pass
 
@@ -709,7 +709,7 @@ def user_activity_intervals() -> Tuple[mark_safe, Dict[str, float]]:
 
     for string_id, realm_intervals in itertools.groupby(all_intervals, by_string_id):
         realm_duration = timedelta(0)
-        output += '<hr>%s\n' % (string_id,)
+        output += f'<hr>{string_id}\n'
         for email, intervals in itertools.groupby(realm_intervals, by_email):
             duration = timedelta(0)
             for interval in intervals:
@@ -723,9 +723,9 @@ def user_activity_intervals() -> Tuple[mark_safe, Dict[str, float]]:
 
         realm_minutes[string_id] = realm_duration.total_seconds() / 60
 
-    output += "\nTotal Duration:                      %s\n" % (total_duration,)
-    output += "\nTotal Duration in minutes:           %s\n" % (total_duration.total_seconds() / 60.,)
-    output += "Total Duration amortized to a month: %s" % (total_duration.total_seconds() * 30. / 60.,)
+    output += f"\nTotal Duration:                      {total_duration}\n"
+    output += f"\nTotal Duration in minutes:           {total_duration.total_seconds() / 60.}\n"
+    output += f"Total Duration amortized to a month: {total_duration.total_seconds() * 30. / 60.}"
     content = mark_safe('<pre>' + output + '</pre>')
     return content, realm_minutes
 
@@ -841,7 +841,7 @@ def ad_hoc_queries() -> List[Dict[str, str]]:
     ###
 
     for mobile_type in ['Android', 'ZulipiOS']:
-        title = '%s usage' % (mobile_type,)
+        title = f'{mobile_type} usage'
 
         query = SQL('''
             select
@@ -1284,13 +1284,13 @@ def format_date_for_activity_reports(date: Optional[datetime]) -> str:
 def user_activity_link(email: str) -> mark_safe:
     url_name = 'analytics.views.get_user_activity'
     url = reverse(url_name, kwargs=dict(email=email))
-    email_link = '<a href="%s">%s</a>' % (url, email)
+    email_link = f'<a href="{url}">{email}</a>'
     return mark_safe(email_link)
 
 def realm_activity_link(realm_str: str) -> mark_safe:
     url_name = 'analytics.views.get_realm_activity'
     url = reverse(url_name, kwargs=dict(realm_str=realm_str))
-    realm_link = '<a href="%s">%s</a>' % (url, realm_str)
+    realm_link = f'<a href="{url}">{realm_str}</a>'
     return mark_safe(realm_link)
 
 def realm_stats_link(realm_str: str) -> mark_safe:
@@ -1449,7 +1449,7 @@ def get_realm_activity(request: HttpRequest, realm_str: str) -> HttpResponse:
     try:
         admins = Realm.objects.get(string_id=realm_str).get_human_admin_users()
     except Realm.DoesNotExist:
-        return HttpResponseNotFound("Realm %s does not exist" % (realm_str,))
+        return HttpResponseNotFound(f"Realm {realm_str} does not exist")
 
     admin_emails = {admin.delivery_email for admin in admins}
 
diff --git a/corporate/lib/stripe.py b/corporate/lib/stripe.py
index 362e13b6be..54ee6d3610 100644
--- a/corporate/lib/stripe.py
+++ b/corporate/lib/stripe.py
@@ -185,7 +185,7 @@ def do_create_stripe_customer(user: UserProfile, stripe_token: Optional[str]=Non
     # bad thing that will happen is that we will create an extra stripe
     # customer that we can delete or ignore.
     stripe_customer = stripe.Customer.create(
-        description="%s (%s)" % (realm.string_id, realm.name),
+        description=f"{realm.string_id} ({realm.name})",
         email=user.delivery_email,
         metadata={'realm_id': realm.id, 'realm_str': realm.string_id},
         source=stripe_token)
diff --git a/corporate/models.py b/corporate/models.py
index b512f7c4ff..bb6ea75fe7 100644
--- a/corporate/models.py
+++ b/corporate/models.py
@@ -14,7 +14,7 @@ class Customer(models.Model):
     default_discount: Optional[Decimal] = models.DecimalField(decimal_places=4, max_digits=7, null=True)
 
     def __str__(self) -> str:
-        return "<Customer %s %s>" % (self.realm, self.stripe_customer_id)
+        return f"<Customer {self.realm} {self.stripe_customer_id}>"
 
 def get_customer_by_realm(realm: Realm) -> Optional[Customer]:
     return Customer.objects.filter(realm=realm).first()
diff --git a/corporate/tests/test_stripe.py b/corporate/tests/test_stripe.py
index d16a8b5c9a..7643ee67f2 100644
--- a/corporate/tests/test_stripe.py
+++ b/corporate/tests/test_stripe.py
@@ -144,7 +144,7 @@ def normalize_fixture_data(decorated_function: CallableT,
     for i, timestamp_field in enumerate(tested_timestamp_fields):
         # Don't use (..) notation, since the matched timestamp can easily appear in other fields
         pattern_translations[
-            '"%s": 1[5-9][0-9]{8}(?![0-9-])' % (timestamp_field,)
+            f'"{timestamp_field}": 1[5-9][0-9]{{8}}(?![0-9-])'
         ] = '"%s": 1%02d%%07d' % (timestamp_field, i+1)
 
     normalized_values: Dict[str, Dict[str, str]] = {
@@ -463,8 +463,8 @@ class StripeTest(StripeTestCase):
         self.assert_not_in_success_response(['Pay annually'], response)
         for substring in [
                 'Zulip Standard', str(self.seat_count),
-                'You are using', '%s of %s licenses' % (self.seat_count, self.seat_count),
-                'Your plan will renew on', 'January 2, 2013', '$%s.00' % (80 * self.seat_count,),
+                'You are using', f'{self.seat_count} of {self.seat_count} licenses',
+                'Your plan will renew on', 'January 2, 2013', f'${80 * self.seat_count}.00',
                 'Visa ending in 4242',
                 'Update card']:
             self.assert_in_response(substring, response)
@@ -547,7 +547,7 @@ class StripeTest(StripeTestCase):
         self.assert_not_in_success_response(['Pay annually', 'Update card'], response)
         for substring in [
                 'Zulip Standard', str(123),
-                'You are using', '%s of %s licenses' % (self.seat_count, 123),
+                'You are using', f'{self.seat_count} of {123} licenses',
                 'Your plan will renew on', 'January 2, 2013', '$9,840.00',  # 9840 = 80 * 123
                 'Billed by invoice']:
             self.assert_in_response(substring, response)
@@ -618,8 +618,8 @@ class StripeTest(StripeTestCase):
             self.assert_not_in_success_response(['Pay annually'], response)
             for substring in [
                     'Zulip Standard', 'Free Trial', str(self.seat_count),
-                    'You are using', '%s of %s licenses' % (self.seat_count, self.seat_count),
-                    'Your plan will be upgraded to', 'March 2, 2012', '$%s.00' % (80 * self.seat_count,),
+                    'You are using', f'{self.seat_count} of {self.seat_count} licenses',
+                    'Your plan will be upgraded to', 'March 2, 2012', f'${80 * self.seat_count}.00',
                     'Visa ending in 4242',
                     'Update card']:
                 self.assert_in_response(substring, response)
@@ -772,7 +772,7 @@ class StripeTest(StripeTestCase):
             self.assert_not_in_success_response(['Pay annually'], response)
             for substring in [
                     'Zulip Standard', 'Free Trial', str(self.seat_count),
-                    'You are using', '%s of %s licenses' % (self.seat_count, 123),
+                    'You are using', f'{self.seat_count} of {123} licenses',
                     'Your plan will be upgraded to', 'March 2, 2012',
                     f'{80 * 123:,.2f}', 'Billed by invoice'
             ]:
diff --git a/corporate/views.py b/corporate/views.py
index 3767a3b139..0f8c8f3ffc 100644
--- a/corporate/views.py
+++ b/corporate/views.py
@@ -114,7 +114,7 @@ def upgrade(request: HttpRequest, user: UserProfile,
             )
         return json_error(e.message, data={'error_description': e.description})
     except Exception as e:
-        billing_logger.exception("Uncaught exception in billing: %s" % (e,))
+        billing_logger.exception(f"Uncaught exception in billing: {e}")
         error_message = BillingError.CONTACT_SUPPORT
         error_description = "uncaught exception during upgrade"
         return json_error(error_message, data={'error_description': error_description})
diff --git a/frontend_tests/run-casper b/frontend_tests/run-casper
index 72d972dd0f..e59ad4f3b6 100755
--- a/frontend_tests/run-casper
+++ b/frontend_tests/run-casper
@@ -121,7 +121,7 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
         for test_file in test_files:
             test_name = os.path.basename(test_file)
             cmd = ["node_modules/.bin/casperjs"] + remote_debug + verbose + xunit_export + ["test", test_file]
-            print("\n\n===================== %s\nRunning %s\n\n" % (test_name, " ".join(map(shlex.quote, cmd))), flush=True)
+            print("\n\n===================== {}\nRunning {}\n\n".format(test_name, " ".join(map(shlex.quote, cmd))), flush=True)
             ret = subprocess.call(cmd)
             if ret != 0:
                 return ret
diff --git a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file
index 416be339a2..64f53c11e7 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file
+++ b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file
@@ -36,7 +36,7 @@ def nagios_from_file(results_file: str, max_time_diff: int=60 * 2) -> 'Tuple[int
             state = pieces[2]
             data = pieces[3]
 
-    return (ret, "%s: %s" % (state, data))
+    return (ret, f"{state}: {data}")
 
 if __name__ == "__main__":
     RESULTS_FILE = sys.argv[1]
diff --git a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_queue_worker_errors b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_queue_worker_errors
index 2a6138bf38..2ed4152bbc 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_queue_worker_errors
+++ b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_queue_worker_errors
@@ -12,7 +12,7 @@ import sys
 wildcard = os.path.join("/var/log/zulip/queue_error", '*.errors')
 clean = True
 for fn in glob.glob(wildcard):
-    print('WARNING: Queue errors logged in %s' % (fn,))
+    print(f'WARNING: Queue errors logged in {fn}')
     clean = False
 
 if not clean:
diff --git a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_rabbitmq_consumers b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_rabbitmq_consumers
index 332e3cef17..c1be7abb78 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_rabbitmq_consumers
+++ b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_rabbitmq_consumers
@@ -17,7 +17,7 @@ if len(sys.argv) < 2:
     print("Please pass the name of the consumer file to check")
     exit(1)
 
-RESULTS_FILE = "/var/lib/nagios_state/check-rabbitmq-consumers-%s" % (sys.argv[1],)
+RESULTS_FILE = f"/var/lib/nagios_state/check-rabbitmq-consumers-{sys.argv[1]}"
 
 ret, result = nagios_from_file(RESULTS_FILE)
 
diff --git a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time
index 6c762d96ff..ba220bded7 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time
+++ b/puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time
@@ -88,24 +88,24 @@ states = {
 def report(state: str, timestamp: Any = None, msg: Optional[str] = None) -> None:
     now = int(time.time())
     if msg is None:
-        msg = "send time was %s" % (timestamp,)
+        msg = f"send time was {timestamp}"
     state_file_path = "/var/lib/nagios_state/check_send_receive_state"
     with open(state_file_path + ".tmp", 'w') as f:
-        f.write("%s|%s|%s|%s\n" % (now, states[state], state, msg))
+        f.write(f"{now}|{states[state]}|{state}|{msg}\n")
     os.rename(state_file_path + ".tmp", state_file_path)
-    print("%s: %s" % (state, msg))
+    print(f"{state}: {msg}")
     exit(states[state])
 
 def send_zulip(sender: zulip.Client, message: Dict[str, Any]) -> None:
     result = sender.send_message(message)
     if result["result"] != "success" and options.nagios:
-        report("CRITICAL", msg="Error sending Zulip, args were: %s, %s" % (message, result))
+        report("CRITICAL", msg=f"Error sending Zulip, args were: {message}, {result}")
 
 def get_zulips() -> List[Dict[str, Any]]:
     global queue_id, last_event_id
     res = zulip_recipient.get_events(queue_id=queue_id, last_event_id=last_event_id)
     if 'error' in res.get('result', {}):
-        report("CRITICAL", msg="Error receiving Zulips, error was: %s" % (res["msg"],))
+        report("CRITICAL", msg="Error receiving Zulips, error was: {}".format(res["msg"]))
     for event in res['events']:
         last_event_id = max(last_event_id, int(event['id']))
     # If we get a heartbeat event, that means we've been hanging for
@@ -141,10 +141,10 @@ zulip_recipient = zulip.Client(
 try:
     res = zulip_recipient.register(event_types=["message"])
     if 'error' in res.get('result', {}):
-        report("CRITICAL", msg="Error subscribing to Zulips: %s" % (res['msg'],))
+        report("CRITICAL", msg="Error subscribing to Zulips: {}".format(res['msg']))
     queue_id, last_event_id = (res['queue_id'], res['last_event_id'])
 except Exception:
-    report("CRITICAL", msg="Error subscribing to Zulips:\n%s" % (traceback.format_exc(),))
+    report("CRITICAL", msg=f"Error subscribing to Zulips:\n{traceback.format_exc()}")
 msg_to_send = str(random.getrandbits(64))
 time_start = time.time()
 
@@ -172,6 +172,6 @@ if options.nagios:
         report('WARNING', timestamp=seconds_diff)
 
 if options.munin:
-    print("sendreceive.value %s" % (seconds_diff,))
+    print(f"sendreceive.value {seconds_diff}")
 elif options.nagios:
     report('OK', timestamp=seconds_diff)
diff --git a/puppet/zulip/files/nagios_plugins/zulip_nagios_server/check_postgres_replication_lag b/puppet/zulip/files/nagios_plugins/zulip_nagios_server/check_postgres_replication_lag
index 5e145f3fca..59b0a3f47a 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_nagios_server/check_postgres_replication_lag
+++ b/puppet/zulip/files/nagios_plugins/zulip_nagios_server/check_postgres_replication_lag
@@ -22,17 +22,17 @@ states = {
 }
 
 def report(state: str, msg: str) -> "NoReturn":
-    print("%s: %s" % (state, msg))
+    print(f"{state}: {msg}")
     exit(states[state])
 
 def get_loc_over_ssh(host: str, func: str) -> str:
     try:
         return subprocess.check_output(['ssh', host,
-                                        'psql -v ON_ERROR_STOP=1 zulip -t -c "SELECT %s()"' % (func,)],
+                                        f'psql -v ON_ERROR_STOP=1 zulip -t -c "SELECT {func}()"'],
                                        stderr=subprocess.STDOUT,
                                        universal_newlines=True)
     except subprocess.CalledProcessError as e:
-        report('CRITICAL', 'ssh failed: %s: %s' % (str(e), e.output))
+        report('CRITICAL', f'ssh failed: {str(e)}: {e.output}')
 
 def loc_to_abs_offset(loc_str: str) -> int:
     m = re.match(r'^\s*([0-9a-fA-F]+)/([0-9a-fA-F]+)\s*$', loc_str)
diff --git a/puppet/zulip/files/nagios_plugins/zulip_postgres_appdb/check_fts_update_log b/puppet/zulip/files/nagios_plugins/zulip_postgres_appdb/check_fts_update_log
index 0571287bdc..1f11d9f60d 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_postgres_appdb/check_fts_update_log
+++ b/puppet/zulip/files/nagios_plugins/zulip_postgres_appdb/check_fts_update_log
@@ -22,7 +22,7 @@ states = {
 }
 
 def report(state: str, num: str) -> None:
-    print("%s: %s rows in fts_update_log table" % (state, num))
+    print(f"{state}: {num} rows in fts_update_log table")
     exit(states[state])
 
 conn = psycopg2.connect(database="zulip")
diff --git a/puppet/zulip/files/nagios_plugins/zulip_postgres_common/check_postgres_backup b/puppet/zulip/files/nagios_plugins/zulip_postgres_common/check_postgres_backup
index 7b7ef54d0c..33305aae70 100755
--- a/puppet/zulip/files/nagios_plugins/zulip_postgres_common/check_postgres_backup
+++ b/puppet/zulip/files/nagios_plugins/zulip_postgres_common/check_postgres_backup
@@ -13,7 +13,7 @@ states = {
 }
 
 def report(state: str, msg: str) -> None:
-    print("%s: %s" % (state, msg))
+    print(f"{state}: {msg}")
     exit(states[state])
 
 if subprocess.check_output(['psql', '-v', 'ON_ERROR_STOP=1',
@@ -28,6 +28,6 @@ except OSError:
     report('UNKNOWN', 'could not determine completion time of last Postgres backup')
 
 if datetime.now(tz=timezone.utc) - last_backup > timedelta(hours=25):
-    report('CRITICAL', 'last Postgres backup completed more than 25 hours ago: %s' % (last_backup,))
+    report('CRITICAL', f'last Postgres backup completed more than 25 hours ago: {last_backup}')
 
-report('OK', 'last Postgres backup completed less than 25 hours ago: %s' % (last_backup,))
+report('OK', f'last Postgres backup completed less than 25 hours ago: {last_backup}')
diff --git a/puppet/zulip/files/postgresql/pg_backup_and_purge b/puppet/zulip/files/postgresql/pg_backup_and_purge
index 1fed9ab152..13844950f3 100755
--- a/puppet/zulip/files/postgresql/pg_backup_and_purge
+++ b/puppet/zulip/files/postgresql/pg_backup_and_purge
@@ -42,7 +42,7 @@ if is_rhel_based:
 else:
     pg_data_paths = glob.glob('/var/lib/postgresql/*/main')
 if len(pg_data_paths) != 1:
-    print("Postgres installation is not unique: %s" % (pg_data_paths,))
+    print(f"Postgres installation is not unique: {pg_data_paths}")
     sys.exit(1)
 pg_data_path = pg_data_paths[0]
 run(['env-wal-e', 'backup-push', pg_data_path])
diff --git a/puppet/zulip/files/postgresql/process_fts_updates b/puppet/zulip/files/postgresql/process_fts_updates
index f0ec736d10..8aedfe20ff 100755
--- a/puppet/zulip/files/postgresql/process_fts_updates
+++ b/puppet/zulip/files/postgresql/process_fts_updates
@@ -142,7 +142,7 @@ while True:
             # Catch up on any historical columns
             while True:
                 rows_updated = update_fts_columns(cursor)
-                notice = "Processed %s rows catching up" % (rows_updated,)
+                notice = f"Processed {rows_updated} rows catching up"
                 if rows_updated > 0:
                     logger.info(notice)
                 else:
diff --git a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_personal_zephyr_mirrors b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_personal_zephyr_mirrors
index 3187b2a5b6..af8fd0ab86 100755
--- a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_personal_zephyr_mirrors
+++ b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_personal_zephyr_mirrors
@@ -22,7 +22,7 @@ states: Dict[str, int] = {
 }
 
 def report(state: str, output: str) -> None:
-    print("%s\n%s" % (state, output))
+    print(f"{state}\n{output}")
     exit(states[state])
 
 output = ""
@@ -41,7 +41,7 @@ for results_file_name in os.listdir(RESULTS_DIR):
         down_count += 1
         this_state = "DOWN"
     last_check_ts = time.strftime("%Y-%m-%d %H:%M %Z", time.gmtime(last_check))
-    output += "%s: %s (%s)\n" % (results_file, this_state, last_check_ts)
+    output += f"{results_file}: {this_state} ({last_check_ts})\n"
 
 if down_count == 0:
     state = "OK"
diff --git a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_user_zephyr_mirror_liveness b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_user_zephyr_mirror_liveness
index d518b6a2ca..c6b525f62a 100755
--- a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_user_zephyr_mirror_liveness
+++ b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_user_zephyr_mirror_liveness
@@ -39,11 +39,11 @@ def report(state: str, short_msg: str, too_old: Optional[Set[Any]] = None) -> No
     too_old_data = ""
     if too_old:
         too_old_data = "\nLast call to get_message for recently out of date mirrors:\n" + "\n".join(
-            ["%16s: %s" % (user.user_profile.email,
-                           user.last_visit.strftime("%Y-%m-%d %H:%M %Z")
-                           ) for user in too_old]
+            ["{:>16}: {}".format(user.user_profile.email,
+                                 user.last_visit.strftime("%Y-%m-%d %H:%M %Z")
+                                 ) for user in too_old]
         )
-    print("%s: %s%s" % (state, short_msg, too_old_data))
+    print(f"{state}: {short_msg}{too_old_data}")
 
     exit(states[state])
 
diff --git a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_zephyr_mirror b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_zephyr_mirror
index fff7cef75a..3e9c86184c 100755
--- a/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_zephyr_mirror
+++ b/puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_zephyr_mirror
@@ -23,7 +23,7 @@ states: Dict[str, int] = {
 }
 
 def report(state: str, data: str, last_check: float) -> None:
-    print("%s: Last test run completed at %s\n%s" % (
+    print("{}: Last test run completed at {}\n{}".format(
         state, time.strftime("%Y-%m-%d %H:%M %Z", time.gmtime(last_check)),
         data))
     exit(states[state])
diff --git a/puppet/zulip_ops/files/zulip-ec2-configure-interfaces b/puppet/zulip_ops/files/zulip-ec2-configure-interfaces
index 9f813dcdc5..d5ecbcf852 100755
--- a/puppet/zulip_ops/files/zulip-ec2-configure-interfaces
+++ b/puppet/zulip_ops/files/zulip-ec2-configure-interfaces
@@ -131,7 +131,7 @@ for device in macs.values():
     for (count, ip) in enumerate(to_configure):
         # Configure the IP via a virtual interface
         device = "ens%i:%i" % (device_number, count)
-        log.info("Configuring %s with IP %s" % (device, ip))
+        log.info(f"Configuring {device} with IP {ip}")
         subprocess.check_call(['/sbin/ifconfig', device, ip])
         subprocess.check_call(
             ['/sbin/iptables', '-t', 'mangle', '-A', 'OUTPUT', '-m', 'conntrack', '--ctorigdst',
diff --git a/scripts/lib/check_rabbitmq_queue.py b/scripts/lib/check_rabbitmq_queue.py
index 34068ab3cc..2bae26d2b5 100644
--- a/scripts/lib/check_rabbitmq_queue.py
+++ b/scripts/lib/check_rabbitmq_queue.py
@@ -77,7 +77,7 @@ def analyze_queue_stats(queue_name: str, stats: Dict[str, Any],
         # 50).
         return dict(status=CRITICAL,
                     name=queue_name,
-                    message='queue appears to be stuck, last update %s, queue size %s' % (
+                    message='queue appears to be stuck, last update {}, queue size {}'.format(
                         stats['update_time'], queue_count_rabbitmqctl))
 
     current_size = stats['current_queue_size']
@@ -108,8 +108,7 @@ def analyze_queue_stats(queue_name: str, stats: Dict[str, Any],
 
             return dict(status=status,
                         name=queue_name,
-                        message='clearing the backlog will take too long: %ss, size: %s' % (
-                            expected_time_to_clear_backlog, current_size))
+                        message=f'clearing the backlog will take too long: {expected_time_to_clear_backlog}s, size: {current_size}')
     else:
         # We slept recently, so treat this as a burst.
         if expected_time_to_clear_backlog > MAX_SECONDS_TO_CLEAR_FOR_BURSTS[queue_name]:
@@ -120,8 +119,7 @@ def analyze_queue_stats(queue_name: str, stats: Dict[str, Any],
 
             return dict(status=status,
                         name=queue_name,
-                        message='clearing the burst will take too long: %ss, size: %s' % (
-                            expected_time_to_clear_backlog, current_size))
+                        message=f'clearing the burst will take too long: {expected_time_to_clear_backlog}s, size: {current_size}')
 
     return dict(status=OK,
                 name=queue_name,
@@ -139,10 +137,10 @@ def check_other_queues(queue_counts_dict: Dict[str, int]) -> List[Dict[str, Any]
 
         if count > CRITICAL_COUNT_THRESHOLD_DEFAULT:
             results.append(dict(status=CRITICAL, name=queue,
-                                message='count critical: %s' % (count,)))
+                                message=f'count critical: {count}'))
         elif count > WARN_COUNT_THRESHOLD_DEFAULT:
             results.append(dict(status=WARNING, name=queue,
-                                message='count warning: %s' % (count,)))
+                                message=f'count warning: {count}'))
         else:
             results.append(dict(status=OK, name=queue, message=''))
 
@@ -210,6 +208,6 @@ def check_rabbitmq_queues() -> None:
             queue_error_template.format(result['name'], states[result['status']], result['message'])
             for result in results if result['status'] > 0
         ])
-        print("%s|%s|%s|%s" % (now, status, states[status], error_message))
+        print(f"{now}|{status}|{states[status]}|{error_message}")
     else:
-        print("%s|%s|%s|queues normal" % (now, status, states[status]))
+        print(f"{now}|{status}|{states[status]}|queues normal")
diff --git a/scripts/lib/sharding.py b/scripts/lib/sharding.py
index 694153f638..df68fbfe09 100755
--- a/scripts/lib/sharding.py
+++ b/scripts/lib/sharding.py
@@ -15,9 +15,9 @@ setup_path()
 from scripts.lib.zulip_tools import get_config_file
 
 def write_realm_nginx_config_line(f: Any, host: str, port: str) -> None:
-    f.write("""if ($host = '%s') {
-    set $tornado_server http://tornado%s;
-}\n""" % (host, port))
+    f.write("""if ($host = '{}') {{
+    set $tornado_server http://tornado{};
+}}\n""".format(host, port))
 
 # Basic system to do Tornado sharding.  Writes two output .tmp files that need
 # to be renamed to the following files to finalize the changes:
@@ -48,7 +48,7 @@ with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
                 host = shard
             else:
                 host = f"{shard}.{external_host}"
-            assert host not in shard_map, "host %s duplicated" % (host,)
+            assert host not in shard_map, f"host {host} duplicated"
             shard_map[host] = int(port)
             write_realm_nginx_config_line(nginx_sharding_conf_f, host, port)
         nginx_sharding_conf_f.write('\n')
diff --git a/scripts/lib/unpack-zulip b/scripts/lib/unpack-zulip
index debde5af7b..4d73e1b257 100755
--- a/scripts/lib/unpack-zulip
+++ b/scripts/lib/unpack-zulip
@@ -14,7 +14,7 @@ from scripts.lib.zulip_tools import DEPLOYMENTS_DIR, FAIL, ENDC, make_deploy_pat
 import version
 
 if len(sys.argv) != 2:
-    print(FAIL + "Usage: %s <tarball>" % (sys.argv[0],) + ENDC)
+    print(FAIL + f"Usage: {sys.argv[0]} <tarball>" + ENDC)
     sys.exit(1)
 
 tarball_path = sys.argv[1]
diff --git a/scripts/lib/upgrade-zulip b/scripts/lib/upgrade-zulip
index 6463c2ddf4..1454b4a0b0 100755
--- a/scripts/lib/upgrade-zulip
+++ b/scripts/lib/upgrade-zulip
@@ -28,12 +28,12 @@ logging.basicConfig(format="%(asctime)s upgrade-zulip: %(message)s",
                     level=logging.INFO)
 
 if len(sys.argv) != 2:
-    print(FAIL + "Usage: %s <tarball>" % (sys.argv[0],) + ENDC)
+    print(FAIL + f"Usage: {sys.argv[0]} <tarball>" + ENDC)
     sys.exit(1)
 
 tarball_path = sys.argv[1]
 
-error_rerun_script = "%s/current/scripts/upgrade-zulip %s" % (DEPLOYMENTS_DIR, tarball_path)
+error_rerun_script = f"{DEPLOYMENTS_DIR}/current/scripts/upgrade-zulip {tarball_path}"
 get_deployment_lock(error_rerun_script)
 
 try:
diff --git a/scripts/lib/upgrade-zulip-from-git b/scripts/lib/upgrade-zulip-from-git
index 2168fb44ec..406354b14b 100755
--- a/scripts/lib/upgrade-zulip-from-git
+++ b/scripts/lib/upgrade-zulip-from-git
@@ -42,7 +42,7 @@ if args.remote_url:
 os.makedirs(DEPLOYMENTS_DIR, exist_ok=True)
 os.makedirs('/home/zulip/logs', exist_ok=True)
 
-error_rerun_script = "%s/current/scripts/upgrade-zulip-from-git %s" % (DEPLOYMENTS_DIR, refname)
+error_rerun_script = f"{DEPLOYMENTS_DIR}/current/scripts/upgrade-zulip-from-git {refname}"
 get_deployment_lock(error_rerun_script)
 
 try:
diff --git a/scripts/nagios/check-rabbitmq-consumers b/scripts/nagios/check-rabbitmq-consumers
index 79688e41cd..59977446e7 100755
--- a/scripts/nagios/check-rabbitmq-consumers
+++ b/scripts/nagios/check-rabbitmq-consumers
@@ -81,7 +81,5 @@ for queue_name in consumers.keys():
     else:
         status = 0
     with open(state_file_tmp, "w") as f:
-        f.write("%s|%s|%s|queue %s has %s consumers, needs %s\n" % (
-            now, status, states[status], queue_name,
-            consumers[queue_name], target_count))
+        f.write(f"{now}|{status}|{states[status]}|queue {queue_name} has {consumers[queue_name]} consumers, needs {target_count}\n")
     os.rename(state_file_tmp, state_file_path)
diff --git a/scripts/nagios/cron_file_helper.py b/scripts/nagios/cron_file_helper.py
index e71e8218f6..f8a7954e3c 100644
--- a/scripts/nagios/cron_file_helper.py
+++ b/scripts/nagios/cron_file_helper.py
@@ -32,4 +32,4 @@ def nagios_from_file(results_file: str) -> Tuple[int, str]:
             state = pieces[2]
             data = pieces[3]
 
-    return (ret, "%s: %s" % (state, data))
+    return (ret, f"{state}: {data}")
diff --git a/scripts/restart-server b/scripts/restart-server
index be2306d643..6a0bb789e5 100755
--- a/scripts/restart-server
+++ b/scripts/restart-server
@@ -66,7 +66,7 @@ if tornado_processes > 1:
         # supervisord group where if any individual process is slow to
         # stop, the whole bundle stays stopped for an extended time.
         logging.info("Restarting Tornado process on port %s", p)
-        subprocess.check_call(["supervisorctl", "restart", "zulip-tornado:port-%s" % (p,)])
+        subprocess.check_call(["supervisorctl", "restart", f"zulip-tornado:port-{p}"])
 else:
     logging.info("Restarting Tornado process")
     subprocess.check_call(["supervisorctl", "restart", "zulip-tornado", "zulip-tornado:*"])
diff --git a/scripts/setup/generate_secrets.py b/scripts/setup/generate_secrets.py
index 1e58718dff..8a8fa111c5 100755
--- a/scripts/setup/generate_secrets.py
+++ b/scripts/setup/generate_secrets.py
@@ -82,7 +82,7 @@ def generate_secrets(development: bool = False) -> None:
         return name not in current_conf
 
     def add_secret(name: str, value: str) -> None:
-        lines.append("%s = %s\n" % (name, value))
+        lines.append(f"{name} = {value}\n")
         current_conf[name] = value
 
     for name in AUTOGENERATED_SETTINGS:
@@ -178,7 +178,7 @@ def generate_secrets(development: bool = False) -> None:
         # the end of the file due to human editing.
         f.write("\n" + "".join(lines))
 
-    print("Generated new secrets in %s." % (OUTPUT_SETTINGS_FILENAME,))
+    print(f"Generated new secrets in {OUTPUT_SETTINGS_FILENAME}.")
 
 if __name__ == '__main__':
 
diff --git a/scripts/setup/inline_email_css.py b/scripts/setup/inline_email_css.py
index ece74f39a7..58bf6bdc3e 100755
--- a/scripts/setup/inline_email_css.py
+++ b/scripts/setup/inline_email_css.py
@@ -78,7 +78,7 @@ def strip_unnecesary_tags(text: str) -> str:
         text = text[start:end]
         return text
     else:
-        raise ValueError("Template does not have %s or %s" % (start_block, end_block))
+        raise ValueError(f"Template does not have {start_block} or {end_block}")
 
 def get_all_templates_from_directory(directory: str) -> Set[str]:
     result = set()
diff --git a/scripts/zulip-puppet-apply b/scripts/zulip-puppet-apply
index 99be427734..672591c44a 100755
--- a/scripts/zulip-puppet-apply
+++ b/scripts/zulip-puppet-apply
@@ -31,7 +31,7 @@ Exec { path => "/usr/sbin:/usr/bin:/sbin:/bin" }
 """
 
 for pclass in re.split(r'\s*,\s*', config.get('machine', 'puppet_classes')):
-    puppet_config += "include %s\n" % (pclass,)
+    puppet_config += f"include {pclass}\n"
 
 # We use the puppet configuration from the same Zulip checkout as this script
 scripts_path = os.path.join(BASE_DIR, "scripts")
diff --git a/static/assets/favicon/generate b/static/assets/favicon/generate
index dca92f569b..9ba478fd64 100755
--- a/static/assets/favicon/generate
+++ b/static/assets/favicon/generate
@@ -7,7 +7,7 @@ import subprocess
 # Open the SVG and find the number text elements using XPath
 tree = ET.parse('orig.svg')
 elems = [tree.getroot().findall(
-    ".//*[@id='%s']/{http://www.w3.org/2000/svg}tspan" % (name,))[0]
+    f".//*[@id='{name}']/{{http://www.w3.org/2000/svg}}tspan")[0]
     for name in ('number_back', 'number_front')]
 
 for i in range(1, 100):
diff --git a/tools/check-issue-labels b/tools/check-issue-labels
index 703c28e537..fea01f1fc7 100755
--- a/tools/check-issue-labels
+++ b/tools/check-issue-labels
@@ -64,7 +64,7 @@ def check_issue_labels() -> None:
             if args.force:
                 response = requests.get(next_page_url)
             else:
-                response = requests.get(next_page_url, headers={'Authorization': 'token %s' % (token,)})
+                response = requests.get(next_page_url, headers={'Authorization': f'token {token}'})
             if response.status_code == 401:
                 sys.exit("Error. Please check the token.")
             if response.status_code == 403:
diff --git a/tools/create-test-api-docs b/tools/create-test-api-docs
index 69b6c473a3..2c313659d2 100755
--- a/tools/create-test-api-docs
+++ b/tools/create-test-api-docs
@@ -65,11 +65,11 @@ def create_single_page(pattern: str, out_dir: str, href: str, calls: List[Call])
             }
             </style>
             ''')
-        f.write('<h3>%s</h3>\n' % (html.escape(pattern),))
+        f.write(f'<h3>{html.escape(pattern)}</h3>\n')
         calls.sort(key=lambda call: call['status_code'])
         for call in calls:
             f.write('<hr>')
-            f.write('\n%s' % (fix_test_name(call['test_name']),))
+            f.write('\n{}'.format(fix_test_name(call['test_name'])))
             f.write('<div class="test">')
             try:
                 f.write(call['url'])
@@ -77,7 +77,7 @@ def create_single_page(pattern: str, out_dir: str, href: str, calls: List[Call])
                 f.write(call['url'].encode('utf8'))
             f.write('<br>\n')
             f.write(call['method'] + '<br>\n')
-            f.write('status code: %s<br>\n' % (call['status_code'],))
+            f.write('status code: {}<br>\n'.format(call['status_code']))
             f.write('<br>')
             f.write('</div>')
 
@@ -136,13 +136,13 @@ def create_user_docs() -> None:
             f.write('<ul>\n')
             for pattern in sorted(groups[name]):
                 href = pattern.replace('/', '-') + '.html'
-                link = '<a href="%s">%s</a>' % (href, html.escape(pattern))
+                link = f'<a href="{href}">{html.escape(pattern)}</a>'
                 f.write('<li>' + link + '</li>\n')
                 create_single_page(pattern, out_dir, href, pattern_dict[pattern])
             f.write('</ul>')
             f.write('\n')
 
-    print('open %s' % (main_page,))
+    print(f'open {main_page}')
 
 
 if __name__ == '__main__':
diff --git a/tools/lib/html_branches.py b/tools/lib/html_branches.py
index 7dfd379dd5..fbfce1a2ec 100644
--- a/tools/lib/html_branches.py
+++ b/tools/lib/html_branches.py
@@ -190,8 +190,8 @@ def build_id_dict(templates: List[str]) -> (Dict[str, List[str]]):
             list_tags = tokenize(text)
         except FormattedException as e:
             raise Exception('''
-                fn: %s
-                %s''' % (fn, e))
+                fn: {}
+                {}'''.format(fn, e))
 
         for tag in list_tags:
             info = get_tag_info(tag)
diff --git a/tools/lib/provision_inner.py b/tools/lib/provision_inner.py
index 9598603f25..cd71b86c19 100755
--- a/tools/lib/provision_inner.py
+++ b/tools/lib/provision_inner.py
@@ -276,7 +276,7 @@ def main(options: argparse.Namespace) -> int:
 
         destroyed = destroy_leaked_test_databases()
         if destroyed:
-            print("Dropped %s stale test databases!" % (destroyed,))
+            print(f"Dropped {destroyed} stale test databases!")
 
     clean_unused_caches()
 
@@ -304,7 +304,7 @@ def main(options: argparse.Namespace) -> int:
             pass
 
     version_file = os.path.join(UUID_VAR_PATH, 'provision_version')
-    print('writing to %s\n' % (version_file,))
+    print(f'writing to {version_file}\n')
     open(version_file, 'w').write(PROVISION_VERSION + '\n')
 
     print()
diff --git a/tools/lib/sanity_check.py b/tools/lib/sanity_check.py
index c5bfc9edf7..dac8209fdb 100644
--- a/tools/lib/sanity_check.py
+++ b/tools/lib/sanity_check.py
@@ -11,7 +11,7 @@ def check_venv(filename: str) -> None:
         ujson
         zulip
     except ImportError:
-        print("You need to run %s inside a Zulip dev environment." % (filename,))
+        print(f"You need to run {filename} inside a Zulip dev environment.")
         user_id = os.getuid()
         user_name = pwd.getpwuid(user_id).pw_name
         if user_name != 'vagrant' and user_name != 'zulipdev':
diff --git a/tools/lib/template_parser.py b/tools/lib/template_parser.py
index fe07cdbec2..311297a7ea 100644
--- a/tools/lib/template_parser.py
+++ b/tools/lib/template_parser.py
@@ -213,8 +213,8 @@ def validate(fn: Optional[str] = None, text: Optional[str] = None, check_indent:
         tokens = tokenize(text)
     except FormattedException as e:
         raise TemplateParserException('''
-            fn: %s
-            %s''' % (fn, e))
+            fn: {}
+            {}'''.format(fn, e))
 
     class State:
         def __init__(self, func: Callable[[Token], None]) -> None:
diff --git a/tools/lib/test_server.py b/tools/lib/test_server.py
index dfdec9bf37..be325233a8 100644
--- a/tools/lib/test_server.py
+++ b/tools/lib/test_server.py
@@ -37,7 +37,7 @@ def assert_server_running(server: "subprocess.Popen[bytes]", log_file: Optional[
     if server.poll() is not None:
         message = 'Server died unexpectedly!'
         if log_file:
-            message += '\nSee %s\n' % (log_file,)
+            message += f'\nSee {log_file}\n'
         raise RuntimeError(message)
 
 def server_is_up(server: "subprocess.Popen[bytes]", log_file: Optional[str]) -> bool:
diff --git a/tools/pretty-print-html b/tools/pretty-print-html
index c949c210e2..d3fb88a4b3 100755
--- a/tools/pretty-print-html
+++ b/tools/pretty-print-html
@@ -5,7 +5,7 @@ import sys
 
 def clean_html(filenames: List[str]) -> None:
     for fn in filenames:
-        print('Prettifying: %s' % (fn,))
+        print(f'Prettifying: {fn}')
         with open(fn) as f:
             html = f.read()
         phtml = pretty_print_html(html)
diff --git a/tools/review b/tools/review
index 2c98a1aa26..24541b4f0b 100755
--- a/tools/review
+++ b/tools/review
@@ -31,14 +31,14 @@ def check_git_pristine() -> None:
 def ensure_on_clean_master() -> None:
     branch = get_git_branch()
     if branch != 'master':
-        exit('You are still on a feature branch: %s' % (branch,))
+        exit(f'You are still on a feature branch: {branch}')
     check_git_pristine()
     run(['git', 'fetch', 'upstream', 'master'])
     run(['git', 'rebase', 'upstream/master'])
 
 def create_pull_branch(pull_id: int) -> None:
     run(['git', 'fetch', 'upstream', 'pull/%d/head' % (pull_id,)])
-    run(['git', 'checkout', '-B', 'review-%s' % (pull_id,), 'FETCH_HEAD'])
+    run(['git', 'checkout', '-B', f'review-{pull_id}', 'FETCH_HEAD'])
     run(['git', 'rebase', 'upstream/master'])
     run(['git', 'log', 'upstream/master..', '--oneline'])
     run(['git', 'diff', 'upstream/master..', '--name-status'])
diff --git a/tools/run-dev.py b/tools/run-dev.py
index 627e0cbf96..1ee92c8475 100755
--- a/tools/run-dev.py
+++ b/tools/run-dev.py
@@ -98,7 +98,7 @@ if options.test:
 else:
     settings_module = "zproject.settings"
 
-manage_args = ['--settings=%s' % (settings_module,)]
+manage_args = [f'--settings={settings_module}']
 os.environ['DJANGO_SETTINGS_MODULE'] = settings_module
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
@@ -158,7 +158,7 @@ def server_processes() -> List[List[str]]:
          './puppet/zulip/files/postgresql/process_fts_updates', '--quiet'],
         ['./manage.py', 'deliver_scheduled_messages'],
         ['/srv/zulip-thumbor-venv/bin/thumbor', '-c', './zthumbor/thumbor.conf',
-         '-p', '%s' % (thumbor_port,)],
+         '-p', f'{thumbor_port}'],
     ]
 
     # NORMAL (but slower) operation:
diff --git a/tools/setup/emoji/build_emoji b/tools/setup/emoji/build_emoji
index 4e78c55b69..23923323bf 100755
--- a/tools/setup/emoji/build_emoji
+++ b/tools/setup/emoji/build_emoji
@@ -79,7 +79,7 @@ def main() -> None:
         # anyway, because EMOJI_CACHE_PATH is created by puppet before
         # build_emoji would be run.
         run_as_root(["mkdir", "-p", EMOJI_CACHE_PATH])
-        run_as_root(["chown", "%s:%s" % (os.getuid(), os.getgid()), EMOJI_CACHE_PATH])
+        run_as_root(["chown", f"{os.getuid()}:{os.getgid()}", EMOJI_CACHE_PATH])
 
     sha1_hexdigest = generate_sha1sum_emoji(ZULIP_PATH)
     source_emoji_dump = os.path.join(EMOJI_CACHE_PATH, sha1_hexdigest, 'emoji')
@@ -107,7 +107,7 @@ def main() -> None:
         os.remove(os.path.join(TARGET_EMOJI_STYLES, filename))
 
 def percent(f: float) -> str:
-    return '%0.3f%%' % (f * 100,)
+    return f'{f * 100:0.3f}%'
 
 def get_square_size(emoji_data: List[Dict[str, Any]]) -> int:
     """
@@ -207,7 +207,7 @@ def generate_sprite_css_files(cache_path: str,
                 'pos_y': percent(emoji["sheet_y"] / (n - 1)),
             }
 
-    SPRITE_CSS_PATH = os.path.join(cache_path, '%s-sprite.css' % (emojiset,))
+    SPRITE_CSS_PATH = os.path.join(cache_path, f'{emojiset}-sprite.css')
     with open(SPRITE_CSS_PATH, 'w') as f:
         f.write(SPRITE_CSS_FILE_TEMPLATE % {'emojiset': emojiset,
                                             'alt_name': alt_name,
diff --git a/tools/setup/emoji/import_emoji_names_from_csv b/tools/setup/emoji/import_emoji_names_from_csv
index e6d0ed78f6..5cf7d16859 100755
--- a/tools/setup/emoji/import_emoji_names_from_csv
+++ b/tools/setup/emoji/import_emoji_names_from_csv
@@ -47,12 +47,12 @@ def load_data(data_file: str) -> List[List[str]]:
 
 def check_uniqueness(emoji_name: str) -> None:
     if emoji_name in emoji_names:
-        raise Exception("Duplicate emoji name: %s" % (emoji_name,))
+        raise Exception(f"Duplicate emoji name: {emoji_name}")
     emoji_names.add(emoji_name)
 
 def check_valid_emoji_name(emoji_name: str) -> None:
     if re.fullmatch("[+-]?[a-z0-9_-]+", emoji_name) is None:
-        raise Exception("Invalid emoji name: %s" % (emoji_name,))
+        raise Exception(f"Invalid emoji name: {emoji_name}")
 
 def check_emoji_names(canonical_name: str, aliases: List[str]) -> None:
     if canonical_name == 'X':
diff --git a/tools/test-backend b/tools/test-backend
index 9e8bce0574..6f7830849d 100755
--- a/tools/test-backend
+++ b/tools/test-backend
@@ -314,7 +314,7 @@ def main() -> None:
                 classname = suite.rsplit('.', 1)[0]
                 rewrite_arguments(classname)
             elif suite[0].isupper():
-                rewrite_arguments('class %s(' % (suite,))
+                rewrite_arguments(f'class {suite}(')
 
         for i, suite in enumerate(args):
             if suite.startswith('test'):
@@ -403,7 +403,7 @@ def main() -> None:
     # We only check the templates if all the tests ran and passed
     if not failures and full_suite and templates_not_rendered:
         missed_count = len(templates_not_rendered)
-        print("\nError: %s templates have no tests!" % (missed_count,))
+        print(f"\nError: {missed_count} templates have no tests!")
         for template in templates_not_rendered:
             print(f'  {template}')
         print("See zerver/tests/test_templates.py for the exclude list.")
@@ -424,8 +424,8 @@ def main() -> None:
         for path in enforce_fully_covered:
             missing_lines = cov.analysis2(path)[3]
             if len(missing_lines) > 0:
-                print("ERROR: %s no longer has complete backend test coverage" % (path,))
-                print("  Lines missing coverage: %s" % (missing_lines,))
+                print(f"ERROR: {path} no longer has complete backend test coverage")
+                print(f"  Lines missing coverage: {missing_lines}")
                 print()
                 failures = True
         if failures:
@@ -439,7 +439,7 @@ def main() -> None:
             try:
                 missing_lines = cov.analysis2(path)[3]
                 if len(missing_lines) == 0 and path != "zerver/lib/migrate.py":
-                    print("ERROR: %s has complete backend test coverage but is still in not_yet_fully_covered." % (path,))
+                    print(f"ERROR: {path} has complete backend test coverage but is still in not_yet_fully_covered.")
                     ok = False
             except coverage.misc.NoSource:
                 continue
@@ -471,7 +471,7 @@ def main() -> None:
 
     removed = remove_test_run_directories()
     if removed:
-        print("Removed %s stale test run directories!" % (removed,))
+        print(f"Removed {removed} stale test run directories!")
 
     # We'll have printed whether tests passed or failed above
     sys.exit(bool(failures))
diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 2320665135..7d3f0aa3a4 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -242,7 +242,7 @@ def run_tests_via_node_js() -> int:
     try:
         ret = subprocess.check_call(command)
     except OSError:
-        print('Bad command: %s' % (command,))
+        print(f'Bad command: {command}')
         raise
     except subprocess.CalledProcessError:
         print('\n** Tests failed, PLEASE FIX! **\n')
@@ -257,8 +257,8 @@ def check_line_coverage(fn: str, line_coverage: Dict[Any, Any], line_mapping: Di
             missing_lines.append(str(actual_line["start"]["line"]))
     if missing_lines:
         if log:
-            print_error("%s no longer has complete node test coverage" % (fn,))
-            print("  Lines missing coverage: %s" % (", ".join(sorted(missing_lines, key=int)),))
+            print_error(f"{fn} no longer has complete node test coverage")
+            print("  Lines missing coverage: {}".format(", ".join(sorted(missing_lines, key=int))))
             print()
         return False
     return True
@@ -288,7 +288,7 @@ def enforce_proper_coverage(coverage_json: Any) -> bool:
         path = ROOT_DIR + "/" + relative_path
         if not (path in coverage_json):
             coverage_lost = True
-            print_error("%s has no node test coverage" % (relative_path,))
+            print_error(f"{relative_path} has no node test coverage")
             continue
         line_coverage = coverage_json[path]['s']
         line_mapping = coverage_json[path]['statementMap']
diff --git a/tools/test-js-with-puppeteer b/tools/test-js-with-puppeteer
index 9f76e29a21..fc2219b67c 100755
--- a/tools/test-js-with-puppeteer
+++ b/tools/test-js-with-puppeteer
@@ -58,7 +58,7 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
         for test_file in test_files:
             test_name = os.path.basename(test_file)
             cmd = ["node"] + [test_file]
-            print("\n\n===================== %s\nRunning %s\n\n" % (test_name, " ".join(map(shlex.quote, cmd))), flush=True)
+            print("\n\n===================== {}\nRunning {}\n\n".format(test_name, " ".join(map(shlex.quote, cmd))), flush=True)
             ret = subprocess.call(cmd)
             if ret != 0:
                 return ret
diff --git a/tools/tests/test_html_branches.py b/tools/tests/test_html_branches.py
index 45dbafad32..7db3609124 100644
--- a/tools/tests/test_html_branches.py
+++ b/tools/tests/test_html_branches.py
@@ -108,14 +108,14 @@ class TestHtmlBranches(unittest.TestCase):
 
         self.assertEqual(set(template_id_dict.keys()), {'below_navbar', 'hello_{{ message }}', 'intro'})
         self.assertEqual(template_id_dict['hello_{{ message }}'], [
-                         'Line 12:%s/tools/tests/test_template_data/test_template1.html' % (ZULIP_PATH,),
-                         'Line 12:%s/tools/tests/test_template_data/test_template2.html' % (ZULIP_PATH,)])
+                         f'Line 12:{ZULIP_PATH}/tools/tests/test_template_data/test_template1.html',
+                         f'Line 12:{ZULIP_PATH}/tools/tests/test_template_data/test_template2.html'])
         self.assertEqual(template_id_dict['intro'], [
-                         'Line 10:%s/tools/tests/test_template_data/test_template1.html' % (ZULIP_PATH,),
-                         'Line 11:%s/tools/tests/test_template_data/test_template1.html' % (ZULIP_PATH,),
-                         'Line 11:%s/tools/tests/test_template_data/test_template2.html' % (ZULIP_PATH,)])
+                         f'Line 10:{ZULIP_PATH}/tools/tests/test_template_data/test_template1.html',
+                         f'Line 11:{ZULIP_PATH}/tools/tests/test_template_data/test_template1.html',
+                         f'Line 11:{ZULIP_PATH}/tools/tests/test_template_data/test_template2.html'])
         self.assertEqual(template_id_dict['below_navbar'], [
-                         'Line 10:%s/tools/tests/test_template_data/test_template2.html' % (ZULIP_PATH,)])
+                         f'Line 10:{ZULIP_PATH}/tools/tests/test_template_data/test_template2.html'])
 
     def test_split_for_id_and_class(self) -> None:
         id1 = "{{ red|blue }}"
diff --git a/tools/update-zuliprc-api-field b/tools/update-zuliprc-api-field
index dfde686911..bc8c28c88e 100755
--- a/tools/update-zuliprc-api-field
+++ b/tools/update-zuliprc-api-field
@@ -65,11 +65,11 @@ for zuliprc_path in zuliprc_paths_list:
                 with open(zuliprc_path, 'w+') as w:
                     zuliprc.write(w)
                 result = 'SUCCESS'
-                reason = 'API field updated for user %s' % (email,)
+                reason = f'API field updated for user {email}'
             except OSError:
                 result = 'FAILURE'
                 reason = 'Writing to file unsuccessful'
         else:
             result = 'SUCCESS'
-            reason = 'API key for user %s is already consistent' % (email,)
+            reason = f'API key for user {email} is already consistent'
     print(f'{zuliprc_path}: {result}: {reason}')
diff --git a/tools/webpack b/tools/webpack
index 4e3ce74980..3672cd82ff 100755
--- a/tools/webpack
+++ b/tools/webpack
@@ -97,9 +97,9 @@ def build_for_most_tests() -> None:
     with open('tools/webpack.assets.json') as json_data:
         for entry in json.load(json_data).keys():
             entries[entry] = [{
-                "name": "%s.js" % (entry,),
-                "publicPath": "http://localhost:3000/webpack-stub/%s-stubentry.js" % (entry,),
-                "path": "/stubfolder/%s-stubfile.js" % (entry,)
+                "name": f"{entry}.js",
+                "publicPath": f"http://localhost:3000/webpack-stub/{entry}-stubentry.js",
+                "path": f"/stubfolder/{entry}-stubfile.js"
             }]
     stat_data = {
         "status": "done",
diff --git a/tools/zulip-export/zulip-export b/tools/zulip-export/zulip-export
index d83b6b592f..3ac431fd34 100755
--- a/tools/zulip-export/zulip-export
+++ b/tools/zulip-export/zulip-export
@@ -81,7 +81,7 @@ for msg in result['messages']:
         msg.pop(k, None)
     messages.append(msg)
 
-filename = "zulip-%s.json" % (options.stream,)
+filename = f"zulip-{options.stream}.json"
 with open(filename, 'wb') as f:
     f.write(json.dumps(messages, indent=0, sort_keys=False).encode('utf-8'))
 print("%d messages exported to %s" % (len(messages), filename,))
diff --git a/zerver/context_processors.py b/zerver/context_processors.py
index d6ae401bb8..c4798cd3a8 100644
--- a/zerver/context_processors.py
+++ b/zerver/context_processors.py
@@ -141,7 +141,7 @@ def zulip_default_context(request: HttpRequest) -> Dict[str, Any]:
         'landing_page_navbar_message': settings.LANDING_PAGE_NAVBAR_MESSAGE,
     }
 
-    context['OPEN_GRAPH_URL'] = '%s%s' % (realm_uri, request.path)
+    context['OPEN_GRAPH_URL'] = f'{realm_uri}{request.path}'
     if realm is not None and realm.icon_source == realm.ICON_UPLOADED:
         context['OPEN_GRAPH_IMAGE'] = urljoin(realm_uri, realm_icon)
 
@@ -175,7 +175,7 @@ def login_context(request: HttpRequest) -> Dict[str, Any]:
     no_auth_enabled = True
     for auth_backend_name in AUTH_BACKEND_NAME_MAP:
         name_lower = auth_backend_name.lower()
-        key = "%s_auth_enabled" % (name_lower,)
+        key = f"{name_lower}_auth_enabled"
         is_enabled = auth_enabled_helper([auth_backend_name], realm)
         context[key] = is_enabled
         if is_enabled:
@@ -191,7 +191,7 @@ def login_context(request: HttpRequest) -> Dict[str, Any]:
         external_authentication_methods = get_external_method_dicts(realm)
     )
     for auth_dict in context['page_params']['external_authentication_methods']:
-        auth_dict['button_id_suffix'] = "auth_button_%s" % (auth_dict['name'],)
+        auth_dict['button_id_suffix'] = "auth_button_{}".format(auth_dict['name'])
 
     return context
 
diff --git a/zerver/data_import/gitter.py b/zerver/data_import/gitter.py
index 01e32c8336..1dd10fa8c2 100644
--- a/zerver/data_import/gitter.py
+++ b/zerver/data_import/gitter.py
@@ -97,7 +97,7 @@ def build_userprofile(timestamp: Any, domain_name: str,
 
 def get_user_email(user_data: ZerverFieldsT, domain_name: str) -> str:
     # TODO Get user email from github
-    email = ("%s@users.noreply.github.com" % (user_data['username'],))
+    email = ("{}@users.noreply.github.com".format(user_data['username']))
     return email
 
 def build_stream_map(timestamp: Any,
@@ -240,14 +240,14 @@ def get_usermentions(message: Dict[str, Any], user_map: Dict[str, int],
     if 'mentions' in message:
         for mention in message['mentions']:
             if mention.get('userId') in user_map:
-                gitter_mention = '@%s' % (mention['screenName'],)
+                gitter_mention = '@{}'.format(mention['screenName'])
                 if mention['screenName'] not in user_short_name_to_full_name:
                     logging.info("Mentioned user %s never sent any messages, so has no full name data",
                                  mention['screenName'])
                     full_name = mention['screenName']
                 else:
                     full_name = user_short_name_to_full_name[mention['screenName']]
-                zulip_mention = ('@**%s**' % (full_name,))
+                zulip_mention = (f'@**{full_name}**')
                 message['text'] = message['text'].replace(gitter_mention, zulip_mention)
 
                 mentioned_user_ids.append(user_map[mention['userId']])
diff --git a/zerver/data_import/import_util.py b/zerver/data_import/import_util.py
index a446fa14ea..5ff147ad1b 100644
--- a/zerver/data_import/import_util.py
+++ b/zerver/data_import/import_util.py
@@ -50,7 +50,7 @@ def build_zerver_realm(realm_id: int, realm_subdomain: str, time: float,
                        other_product: str) -> List[ZerverFieldsT]:
     realm = Realm(id=realm_id, date_created=time,
                   name=realm_subdomain, string_id=realm_subdomain,
-                  description="Organization imported from %s!" % (other_product,))
+                  description=f"Organization imported from {other_product}!")
     auth_methods = [[flag[0], flag[1]] for flag in realm.authentication_methods]
     realm_dict = model_to_dict(realm, exclude='authentication_methods')
     realm_dict['authentication_methods'] = auth_methods
@@ -514,8 +514,8 @@ def process_avatars(avatar_list: List[ZerverFieldsT], avatar_dir: str, realm_id:
         avatar_url = avatar['path']
         avatar_original = dict(avatar)
 
-        image_path = '%s.png' % (avatar_hash,)
-        original_image_path = '%s.original' % (avatar_hash,)
+        image_path = f'{avatar_hash}.png'
+        original_image_path = f'{avatar_hash}.original'
 
         avatar_upload_list.append([avatar_url, image_path, original_image_path])
         # We don't add the size field here in avatar's records.json,
diff --git a/zerver/data_import/slack.py b/zerver/data_import/slack.py
index f612fa4d0e..47b0948758 100755
--- a/zerver/data_import/slack.py
+++ b/zerver/data_import/slack.py
@@ -227,7 +227,7 @@ def build_customprofile_field(customprofile_field: List[ZerverFieldsT], fields:
             if field in slack_custom_fields:
                 field_name = field
             else:
-                field_name = "slack custom field %s" % (str(custom_profile_field_id + 1),)
+                field_name = f"slack custom field {str(custom_profile_field_id + 1)}"
             customprofilefield = CustomProfileField(
                 id=custom_profile_field_id,
                 name=field_name,
@@ -294,10 +294,10 @@ def get_user_email(user: ZerverFieldsT, domain_name: str) -> str:
             slack_bot_name = user['profile']['first_name']
         else:
             raise AssertionError("Could not identify bot type")
-        return slack_bot_name.replace("Bot", "").replace(" ", "") + "-bot@%s" % (domain_name,)
+        return slack_bot_name.replace("Bot", "").replace(" ", "") + f"-bot@{domain_name}"
     if get_user_full_name(user).lower() == "slackbot":
-        return "imported-slackbot-bot@%s" % (domain_name,)
-    raise AssertionError("Could not find email address for Slack user %s" % (user,))
+        return f"imported-slackbot-bot@{domain_name}"
+    raise AssertionError(f"Could not find email address for Slack user {user}")
 
 def build_avatar_url(slack_user_id: str, team_id: str, avatar_hash: str) -> str:
     avatar_url = f"https://ca.slack-edge.com/{team_id}-{slack_user_id}-{avatar_hash}"
@@ -742,7 +742,7 @@ def channel_message_to_zerver_message(realm_id: int,
         # For example "sh_room_created" has the message 'started a call'
         # which should be displayed as '/me started a call'
         if subtype in ["bot_add", "sh_room_created", "me_message"]:
-            content = '/me %s' % (content,)
+            content = f'/me {content}'
         if subtype == 'file_comment':
             # The file_comment message type only indicates the
             # responsible user in a subfield.
@@ -864,7 +864,7 @@ def process_message_files(message: ZerverFieldsT,
                 file_name = fileinfo['title']
             else:
                 file_name = fileinfo['name']
-            markdown_links.append('[%s](%s)' % (file_name, fileinfo['url_private']))
+            markdown_links.append('[{}]({})'.format(file_name, fileinfo['url_private']))
 
     content = '\n'.join(markdown_links)
 
@@ -887,8 +887,8 @@ def get_attachment_path_and_content(fileinfo: ZerverFieldsT, realm_id: int) -> T
         random_name(18),
         sanitize_name(fileinfo['name'])
     ])
-    attachment_path = '/user_uploads/%s' % (s3_path,)
-    content = '[%s](%s)' % (fileinfo['title'], attachment_path)
+    attachment_path = f'/user_uploads/{s3_path}'
+    content = '[{}]({})'.format(fileinfo['title'], attachment_path)
 
     return s3_path, content
 
@@ -1119,7 +1119,7 @@ def get_slack_api_data(slack_api_url: str, get_param: str, **kwargs: Any) -> Any
     if data.status_code == requests.codes.ok:
         result = data.json()
         if not result['ok']:
-            raise Exception('Error accessing Slack API: %s' % (result['error'],))
+            raise Exception('Error accessing Slack API: {}'.format(result['error']))
         return result[get_param]
 
     raise Exception('HTTP error accessing the Slack API.')
diff --git a/zerver/data_import/slack_message_conversion.py b/zerver/data_import/slack_message_conversion.py
index e0aefcca5b..0905541568 100644
--- a/zerver/data_import/slack_message_conversion.py
+++ b/zerver/data_import/slack_message_conversion.py
@@ -85,7 +85,7 @@ def convert_to_zulip_markdown(text: str, users: List[ZerverFieldsT],
     # Map Slack channel mention: '<#C5Z73A7RA|general>' to '#**general**'
     for cname, ids in added_channels.items():
         cid = ids[0]
-        text = text.replace('<#%s|%s>' % (cid, cname), '#**' + cname + '**')
+        text = text.replace(f'<#{cid}|{cname}>', '#**' + cname + '**')
 
     tokens = text.split(' ')
     for iterator in range(len(tokens)):
diff --git a/zerver/forms.py b/zerver/forms.py
index 617dcea6ca..b70bf414c3 100644
--- a/zerver/forms.py
+++ b/zerver/forms.py
@@ -57,7 +57,7 @@ def email_is_not_mit_mailing_list(email: str) -> None:
         username = email.rsplit("@", 1)[0]
         # Check whether the user exists and can get mail.
         try:
-            DNS.dnslookup("%s.pobox.ns.athena.mit.edu" % (username,), DNS.Type.TXT)
+            DNS.dnslookup(f"{username}.pobox.ns.athena.mit.edu", DNS.Type.TXT)
         except DNS.Base.ServerError as e:
             if e.rcode == DNS.Status.NXDOMAIN:
                 raise ValidationError(mark_safe(MIT_VALIDATION_ERROR))
diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index 9fb9308231..a32462f9c7 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -394,10 +394,7 @@ def process_new_human_user(user_profile: UserProfile,
             user_profile.realm,
             get_system_bot(settings.NOTIFICATION_BOT),
             prereg_user.referred_by,
-            "%s <`%s`> accepted your invitation to join Zulip!" % (
-                user_profile.full_name,
-                user_profile.email,
-            )
+            f"{user_profile.full_name} <`{user_profile.email}`> accepted your invitation to join Zulip!"
         )
     # Mark any other PreregistrationUsers that are STATUS_ACTIVE as
     # inactive so we can keep track of the PreregistrationUser we
@@ -598,8 +595,7 @@ def do_set_realm_property(realm: Realm, name: str, value: Any) -> None:
     """
     property_type = Realm.property_types[name]
     assert isinstance(value, property_type), (
-        'Cannot update %s: %s is not an instance of %s' % (
-            name, value, property_type,))
+        f'Cannot update {name}: {value} is not an instance of {property_type}')
 
     old_value = getattr(realm, name)
     setattr(realm, name, value)
@@ -2359,8 +2355,7 @@ def _internal_prep_message(realm: Realm,
         return check_message(sender, get_client("Internal"), addressee,
                              content, realm=realm)
     except JsonableError as e:
-        logging.exception("Error queueing internal message by %s: %s" % (
-            sender.delivery_email, e))
+        logging.exception(f"Error queueing internal message by {sender.delivery_email}: {e}")
 
     return None
 
@@ -3587,7 +3582,7 @@ def do_change_stream_description(stream: Stream, new_description: str) -> None:
 def do_create_realm(string_id: str, name: str,
                     emails_restricted_to_domains: Optional[bool]=None) -> Realm:
     if Realm.objects.filter(string_id=string_id).exists():
-        raise AssertionError("Realm %s already exists!" % (string_id,))
+        raise AssertionError(f"Realm {string_id} already exists!")
     if not server_initialized():
         logging.info("Server not yet initialized. Creating the internal realm first.")
         create_internal_realm()
@@ -3653,8 +3648,7 @@ def do_change_notification_settings(user_profile: UserProfile, name: str,
 
     notification_setting_type = UserProfile.notification_setting_types[name]
     assert isinstance(value, notification_setting_type), (
-        'Cannot update %s: %s is not an instance of %s' % (
-            name, value, notification_setting_type,))
+        f'Cannot update {name}: {value} is not an instance of {notification_setting_type}')
 
     setattr(user_profile, name, value)
 
@@ -4179,7 +4173,7 @@ def do_update_message_flags(user_profile: UserProfile,
     if flag == "read" and operation == "add":
         do_clear_mobile_push_notifications_for_ids(user_profile, messages)
 
-    statsd.incr("flags.%s.%s" % (flag, operation), count)
+    statsd.incr(f"flags.{flag}.{operation}", count)
     return count
 
 class MessageUpdateUserInfoResult(TypedDict):
@@ -4199,9 +4193,9 @@ def notify_topic_moved_streams(user_profile: UserProfile,
     if new_topic is None:
         new_topic = old_topic
 
-    user_mention = "@_**%s|%s**" % (user_profile.full_name, user_profile.id)
-    old_topic_link = "#**%s>%s**" % (old_stream.name, old_topic)
-    new_topic_link = "#**%s>%s**" % (new_stream.name, new_topic)
+    user_mention = f"@_**{user_profile.full_name}|{user_profile.id}**"
+    old_topic_link = f"#**{old_stream.name}>{old_topic}**"
+    new_topic_link = f"#**{new_stream.name}>{new_topic}**"
     if send_notification_to_new_thread:
         internal_send_stream_message(
             new_stream.realm, sender, new_stream, new_topic,
@@ -4918,7 +4912,7 @@ def do_send_confirmation_email(invitee: PreregistrationUser,
     activation_url = create_confirmation_link(invitee, referrer.realm.host, Confirmation.INVITATION)
     context = {'referrer_full_name': referrer.full_name, 'referrer_email': referrer.delivery_email,
                'activate_url': activation_url, 'referrer_realm_name': referrer.realm.name}
-    from_name = "%s (via Zulip)" % (referrer.full_name,)
+    from_name = f"{referrer.full_name} (via Zulip)"
     send_email('zerver/emails/invitation', to_emails=[invitee.email], from_name=from_name,
                from_address=FromAddress.tokenized_no_reply_address(),
                language=referrer.realm.default_language, context=context)
diff --git a/zerver/lib/avatar.py b/zerver/lib/avatar.py
index 9078fbeca8..469b8e27f8 100644
--- a/zerver/lib/avatar.py
+++ b/zerver/lib/avatar.py
@@ -92,9 +92,9 @@ def get_gravatar_url(email: str, avatar_version: int, medium: bool=False) -> str
 
 def _get_unversioned_gravatar_url(email: str, medium: bool) -> str:
     if settings.ENABLE_GRAVATAR:
-        gravitar_query_suffix = "&s=%s" % (MEDIUM_AVATAR_SIZE,) if medium else ""
+        gravitar_query_suffix = f"&s={MEDIUM_AVATAR_SIZE}" if medium else ""
         hash_key = gravatar_hash(email)
-        return "https://secure.gravatar.com/avatar/%s?d=identicon%s" % (hash_key, gravitar_query_suffix)
+        return f"https://secure.gravatar.com/avatar/{hash_key}?d=identicon{gravitar_query_suffix}"
     return settings.DEFAULT_AVATAR_URI+'?x=x'
 
 def _get_unversioned_avatar_url(user_profile_id: int,
diff --git a/zerver/lib/avatar_hash.py b/zerver/lib/avatar_hash.py
index ac645c9614..bbd6c10c17 100644
--- a/zerver/lib/avatar_hash.py
+++ b/zerver/lib/avatar_hash.py
@@ -34,7 +34,7 @@ def user_avatar_path(user_profile: UserProfile) -> str:
 
 def user_avatar_path_from_ids(user_profile_id: int, realm_id: int) -> str:
     user_id_hash = user_avatar_hash(str(user_profile_id))
-    return '%s/%s' % (str(realm_id), user_id_hash)
+    return f'{str(realm_id)}/{user_id_hash}'
 
 def user_avatar_content_hash(ldap_avatar: bytes) -> str:
     return hashlib.sha256(ldap_avatar).hexdigest()
diff --git a/zerver/lib/bot_lib.py b/zerver/lib/bot_lib.py
index 37532e3890..24532c3f67 100644
--- a/zerver/lib/bot_lib.py
+++ b/zerver/lib/bot_lib.py
@@ -27,7 +27,7 @@ def get_bot_handler(service_name: str) -> Any:
             configured_service = embedded_bot_service.name
     if not configured_service:
         return None
-    bot_module_name = 'zulip_bots.bots.%s.%s' % (configured_service, configured_service)
+    bot_module_name = f'zulip_bots.bots.{configured_service}.{configured_service}'
     bot_module: Any = importlib.import_module(bot_module_name)
     return bot_module.handler_class()
 
diff --git a/zerver/lib/bugdown/help_relative_links.py b/zerver/lib/bugdown/help_relative_links.py
index 1aa1251e0c..910d908471 100644
--- a/zerver/lib/bugdown/help_relative_links.py
+++ b/zerver/lib/bugdown/help_relative_links.py
@@ -34,9 +34,9 @@ gear_instructions = """
 
 def gear_handle_match(key: str) -> str:
     if relative_help_links:
-        item = '[%s](%s)' % (gear_info[key][0], gear_info[key][1])
+        item = f'[{gear_info[key][0]}]({gear_info[key][1]})'
     else:
-        item = '**%s**' % (gear_info[key][0],)
+        item = f'**{gear_info[key][0]}**'
     return gear_instructions % {'item': item}
 
 
@@ -54,7 +54,7 @@ stream_instructions_no_link = """
 
 def stream_handle_match(key: str) -> str:
     if relative_help_links:
-        return "1. Go to [%s](%s)." % (stream_info[key][0], stream_info[key][1])
+        return f"1. Go to [{stream_info[key][0]}]({stream_info[key][1]})."
     if key == 'all':
         return stream_instructions_no_link + "\n\n1. Click **All streams** in the upper left."
     return stream_instructions_no_link
diff --git a/zerver/lib/bugdown/help_settings_links.py b/zerver/lib/bugdown/help_settings_links.py
index b313909a60..4c7b9e34e3 100644
--- a/zerver/lib/bugdown/help_settings_links.py
+++ b/zerver/lib/bugdown/help_settings_links.py
@@ -103,9 +103,9 @@ class Setting(Preprocessor):
         setting_name = link_mapping[setting_identifier][1]
         setting_link = link_mapping[setting_identifier][2]
         if relative_settings_links:
-            return "1. Go to [%s](%s)." % (setting_name, setting_link)
+            return f"1. Go to [{setting_name}]({setting_link})."
         return settings_markdown % {'setting_type_name': setting_type_name,
-                                    'setting_reference': "**%s**" % (setting_name,)}
+                                    'setting_reference': f"**{setting_name}**"}
 
 def makeExtension(*args: Any, **kwargs: Any) -> SettingHelpExtension:
     return SettingHelpExtension(*args, **kwargs)
diff --git a/zerver/lib/cache.py b/zerver/lib/cache.py
index 866ab5b617..1a1a22ca09 100644
--- a/zerver/lib/cache.py
+++ b/zerver/lib/cache.py
@@ -176,7 +176,7 @@ def cache_with_key(
                 metric_key = statsd_key(key)
 
             status = "hit" if val is not None else "miss"
-            statsd.incr("cache%s.%s.%s" % (extra, metric_key, status))
+            statsd.incr(f"cache{extra}.{metric_key}.{status}")
 
             # Values are singleton tuples so that we can distinguish
             # a result of None from a missing key.
@@ -409,7 +409,7 @@ def generic_bulk_cached_fetch(
             if cache_keys[object_id] in cached_objects}
 
 def preview_url_cache_key(url: str) -> str:
-    return "preview_url:%s" % (make_safe_digest(url),)
+    return f"preview_url:{make_safe_digest(url)}"
 
 def display_recipient_cache_key(recipient_id: int) -> str:
     return "display_recipient_dict:%d" % (recipient_id,)
@@ -423,22 +423,22 @@ def user_profile_by_email_cache_key(email: str) -> str:
     # See the comment in zerver/lib/avatar_hash.py:gravatar_hash for why we
     # are proactively encoding email addresses even though they will
     # with high likelihood be ASCII-only for the foreseeable future.
-    return 'user_profile_by_email:%s' % (make_safe_digest(email.strip()),)
+    return f'user_profile_by_email:{make_safe_digest(email.strip())}'
 
 def user_profile_cache_key_id(email: str, realm_id: int) -> str:
-    return "user_profile:%s:%s" % (make_safe_digest(email.strip()), realm_id,)
+    return f"user_profile:{make_safe_digest(email.strip())}:{realm_id}"
 
 def user_profile_cache_key(email: str, realm: 'Realm') -> str:
     return user_profile_cache_key_id(email, realm.id)
 
 def bot_profile_cache_key(email: str) -> str:
-    return "bot_profile:%s" % (make_safe_digest(email.strip()),)
+    return f"bot_profile:{make_safe_digest(email.strip())}"
 
 def user_profile_by_id_cache_key(user_profile_id: int) -> str:
-    return "user_profile_by_id:%s" % (user_profile_id,)
+    return f"user_profile_by_id:{user_profile_id}"
 
 def user_profile_by_api_key_cache_key(api_key: str) -> str:
-    return "user_profile_by_api_key:%s" % (api_key,)
+    return f"user_profile_by_api_key:{api_key}"
 
 realm_user_dict_fields: List[str] = [
     'id', 'full_name', 'short_name', 'email',
@@ -449,16 +449,16 @@ realm_user_dict_fields: List[str] = [
 ]
 
 def realm_user_dicts_cache_key(realm_id: int) -> str:
-    return "realm_user_dicts:%s" % (realm_id,)
+    return f"realm_user_dicts:{realm_id}"
 
 def get_realm_used_upload_space_cache_key(realm: 'Realm') -> str:
-    return 'realm_used_upload_space:%s' % (realm.id,)
+    return f'realm_used_upload_space:{realm.id}'
 
 def active_user_ids_cache_key(realm_id: int) -> str:
-    return "active_user_ids:%s" % (realm_id,)
+    return f"active_user_ids:{realm_id}"
 
 def active_non_guest_user_ids_cache_key(realm_id: int) -> str:
-    return "active_non_guest_user_ids:%s" % (realm_id,)
+    return f"active_non_guest_user_ids:{realm_id}"
 
 bot_dict_fields: List[str] = [
     'api_key',
@@ -478,11 +478,10 @@ bot_dict_fields: List[str] = [
 ]
 
 def bot_dicts_in_realm_cache_key(realm: 'Realm') -> str:
-    return "bot_dicts_in_realm:%s" % (realm.id,)
+    return f"bot_dicts_in_realm:{realm.id}"
 
 def get_stream_cache_key(stream_name: str, realm_id: int) -> str:
-    return "stream_by_realm_and_name:%s:%s" % (
-        realm_id, make_safe_digest(stream_name.strip().lower()))
+    return f"stream_by_realm_and_name:{realm_id}:{make_safe_digest(stream_name.strip().lower())}"
 
 def delete_user_profile_caches(user_profiles: Iterable['UserProfile']) -> None:
     # Imported here to avoid cyclic dependency.
@@ -571,16 +570,16 @@ def flush_realm(sender: Any, **kwargs: Any) -> None:
         cache_delete(realm_text_description_cache_key(realm))
 
 def realm_alert_words_cache_key(realm: 'Realm') -> str:
-    return "realm_alert_words:%s" % (realm.string_id,)
+    return f"realm_alert_words:{realm.string_id}"
 
 def realm_alert_words_automaton_cache_key(realm: 'Realm') -> str:
-    return "realm_alert_words_automaton:%s" % (realm.string_id,)
+    return f"realm_alert_words_automaton:{realm.string_id}"
 
 def realm_rendered_description_cache_key(realm: 'Realm') -> str:
-    return "realm_rendered_description:%s" % (realm.string_id,)
+    return f"realm_rendered_description:{realm.string_id}"
 
 def realm_text_description_cache_key(realm: 'Realm') -> str:
-    return "realm_text_description:%s" % (realm.string_id,)
+    return f"realm_text_description:{realm.string_id}"
 
 # Called by models.py to flush the stream cache whenever we save a stream
 # object.
@@ -610,7 +609,7 @@ def to_dict_cache_key(message: 'Message', realm_id: Optional[int]=None) -> str:
     return to_dict_cache_key_id(message.id)
 
 def open_graph_description_cache_key(content: Any, request: HttpRequest) -> str:
-    return 'open_graph_description_path:%s' % (make_safe_digest(request.META['PATH_INFO']),)
+    return 'open_graph_description_path:{}'.format(make_safe_digest(request.META['PATH_INFO']))
 
 def flush_message(sender: Any, **kwargs: Any) -> None:
     message = kwargs['instance']
diff --git a/zerver/lib/camo.py b/zerver/lib/camo.py
index 0079893ba0..d1c7a66f1c 100644
--- a/zerver/lib/camo.py
+++ b/zerver/lib/camo.py
@@ -8,7 +8,7 @@ def generate_camo_url(url: str) -> str:
     encoded_camo_key = settings.CAMO_KEY.encode("utf-8")
     digest = hmac.new(encoded_camo_key, encoded_url, hashlib.sha1).hexdigest()
     hex_encoded_url = binascii.b2a_hex(encoded_url)
-    return "%s/%s" % (digest, hex_encoded_url.decode("utf-8"))
+    return "{}/{}".format(digest, hex_encoded_url.decode("utf-8"))
 
 # Encodes the provided URL using the same algorithm used by the camo
 # caching https image proxy
@@ -16,7 +16,7 @@ def get_camo_url(url: str) -> str:
     # Only encode the url if Camo is enabled
     if settings.CAMO_URI == '':
         return url
-    return "%s%s" % (settings.CAMO_URI, generate_camo_url(url))
+    return f"{settings.CAMO_URI}{generate_camo_url(url)}"
 
 def is_camo_url_valid(digest: str, url: str) -> bool:
     camo_url = generate_camo_url(url)
diff --git a/zerver/lib/create_user.py b/zerver/lib/create_user.py
index c5e1ad5755..7924db0bbb 100644
--- a/zerver/lib/create_user.py
+++ b/zerver/lib/create_user.py
@@ -33,7 +33,7 @@ def copy_user_settings(source_profile: UserProfile, target_profile: UserProfile)
 
 def get_display_email_address(user_profile: UserProfile, realm: Realm) -> str:
     if not user_profile.email_address_is_realm_public():
-        return "user%s@%s" % (user_profile.id, get_fake_email_domain())
+        return f"user{user_profile.id}@{get_fake_email_domain()}"
     return user_profile.delivery_email
 
 def get_role_for_new_user(invited_as: int, realm_creation: bool=False) -> int:
diff --git a/zerver/lib/db.py b/zerver/lib/db.py
index 038064b86a..2971ac2edc 100644
--- a/zerver/lib/db.py
+++ b/zerver/lib/db.py
@@ -23,7 +23,7 @@ def wrapper_execute(self: CursorObj,
         stop = time.time()
         duration = stop - start
         self.connection.queries.append({
-            'time': "%.3f" % (duration,),
+            'time': f"{duration:.3f}",
         })
 
 class TimeTrackingCursor(cursor):
diff --git a/zerver/lib/digest.py b/zerver/lib/digest.py
index d4192644f8..f5e15a8c6f 100644
--- a/zerver/lib/digest.py
+++ b/zerver/lib/digest.py
@@ -146,14 +146,14 @@ def gather_new_streams(user_profile: UserProfile,
     else:
         new_streams = []
 
-    base_url = "%s/#narrow/stream/" % (user_profile.realm.uri,)
+    base_url = f"{user_profile.realm.uri}/#narrow/stream/"
 
     streams_html = []
     streams_plain = []
 
     for stream in new_streams:
         narrow_url = base_url + encode_stream(stream.id, stream.name)
-        stream_link = "<a href='%s'>%s</a>" % (narrow_url, stream.name)
+        stream_link = f"<a href='{narrow_url}'>{stream.name}</a>"
         streams_html.append(stream_link)
         streams_plain.append(stream.name)
 
diff --git a/zerver/lib/email_mirror.py b/zerver/lib/email_mirror.py
index d91ed3b310..426529965e 100644
--- a/zerver/lib/email_mirror.py
+++ b/zerver/lib/email_mirror.py
@@ -69,7 +69,7 @@ def report_to_zulip(error_message: str) -> None:
         error_bot,
         error_stream,
         "email mirror error",
-        """~~~\n%s\n~~~""" % (error_message,)
+        f"""~~~\n{error_message}\n~~~"""
     )
 
 def log_and_report(email_message: EmailMessage, error_message: str, to: Optional[str]) -> None:
@@ -155,7 +155,7 @@ def construct_zulip_body(message: EmailMessage, realm: Realm, show_sender: bool=
 
     if show_sender:
         sender = handle_header_content(message.get("From", ""))
-        body = "From: %s\n%s" % (sender, body)
+        body = f"From: {sender}\n{body}"
 
     return body
 
@@ -279,7 +279,7 @@ def extract_and_upload_attachments(message: EmailMessage, realm: Realm) -> str:
                                              attachment,
                                              user_profile,
                                              target_realm=realm)
-                formatted_link = "[%s](%s)" % (filename, s3_url)
+                formatted_link = f"[{filename}]({s3_url})"
                 attachment_links.append(formatted_link)
             else:
                 logger.warning("Payload is not bytes (invalid attachment %s in message from %s).",
diff --git a/zerver/lib/email_mirror_helpers.py b/zerver/lib/email_mirror_helpers.py
index dabbcb5503..f79ae02ac8 100644
--- a/zerver/lib/email_mirror_helpers.py
+++ b/zerver/lib/email_mirror_helpers.py
@@ -60,7 +60,7 @@ def encode_email_address_helper(name: str, email_token: str, show_sender: bool=F
 
     # If encoded_name ends up empty, we just skip this part of the address:
     if encoded_name:
-        encoded_token = "%s.%s" % (encoded_name, email_token)
+        encoded_token = f"{encoded_name}.{email_token}"
     else:
         encoded_token = email_token
 
diff --git a/zerver/lib/email_notifications.py b/zerver/lib/email_notifications.py
index a95ce07453..70bf7f8c50 100644
--- a/zerver/lib/email_notifications.py
+++ b/zerver/lib/email_notifications.py
@@ -93,16 +93,9 @@ def fix_emojis(content: str, base_url: str, emojiset: str) -> str:
         emoji_code = match.group('emoji_code')
         emoji_name = emoji_span_elem.get('title')
         alt_code = emoji_span_elem.text
-        image_url = base_url + '/static/generated/emoji/images-%(emojiset)s-64/%(emoji_code)s.png' % {
-            'emojiset': emojiset,
-            'emoji_code': emoji_code
-        }
+        image_url = base_url + f'/static/generated/emoji/images-{emojiset}-64/{emoji_code}.png'
         img_elem = lxml.html.fromstring(
-            '<img alt="%(alt_code)s" src="%(image_url)s" title="%(title)s">' % {
-                'alt_code': alt_code,
-                'image_url': image_url,
-                'title': emoji_name,
-            })
+            f'<img alt="{alt_code}" src="{image_url}" title="{emoji_name}">')
         img_elem.set('style', 'height: 20px;')
         img_elem.tail = emoji_span_elem.tail
         return img_elem
@@ -179,8 +172,8 @@ def build_message_list(user_profile: UserProfile, messages: List[Message]) -> Li
     def message_header(user_profile: UserProfile, message: Message) -> Dict[str, Any]:
         if message.recipient.type == Recipient.PERSONAL:
             narrow_link = get_narrow_url(user_profile, message)
-            header = "You and %s" % (message.sender.full_name,)
-            header_html = "<a style='color: #ffffff;' href='%s'>%s</a>" % (narrow_link, header)
+            header = f"You and {message.sender.full_name}"
+            header_html = f"<a style='color: #ffffff;' href='{narrow_link}'>{header}</a>"
         elif message.recipient.type == Recipient.HUDDLE:
             display_recipient = get_display_recipient(message.recipient)
             assert not isinstance(display_recipient, str)
@@ -188,15 +181,14 @@ def build_message_list(user_profile: UserProfile, messages: List[Message]) -> Li
                                          display_recipient=display_recipient)
             other_recipients = [r['full_name'] for r in display_recipient
                                 if r['id'] != user_profile.id]
-            header = "You and %s" % (", ".join(other_recipients),)
-            header_html = "<a style='color: #ffffff;' href='%s'>%s</a>" % (narrow_link, header)
+            header = "You and {}".format(", ".join(other_recipients))
+            header_html = f"<a style='color: #ffffff;' href='{narrow_link}'>{header}</a>"
         else:
             stream = Stream.objects.only('id', 'name').get(id=message.recipient.type_id)
             narrow_link = get_narrow_url(user_profile, message, stream=stream)
-            header = "%s > %s" % (stream.name, message.topic_name())
+            header = f"{stream.name} > {message.topic_name()}"
             stream_link = stream_narrow_url(user_profile.realm, stream)
-            header_html = "<a href='%s'>%s</a> > <a href='%s'>%s</a>" % (
-                stream_link, stream.name, narrow_link, message.topic_name())
+            header_html = f"<a href='{stream_link}'>{stream.name}</a> > <a href='{narrow_link}'>{message.topic_name()}</a>"
         return {"plain": header,
                 "html": header_html,
                 "stream_message": message.recipient.type_name() == "stream"}
@@ -371,11 +363,10 @@ def do_send_missedmessage_events_reply_in_zulip(user_profile: UserProfile,
             huddle_display_name = " and ".join(other_recipients)
             context.update({'huddle_display_name': huddle_display_name})
         elif len(other_recipients) == 3:
-            huddle_display_name = "%s, %s, and %s" % (
-                other_recipients[0], other_recipients[1], other_recipients[2])
+            huddle_display_name = f"{other_recipients[0]}, {other_recipients[1]}, and {other_recipients[2]}"
             context.update({'huddle_display_name': huddle_display_name})
         else:
-            huddle_display_name = "%s, and %s others" % (
+            huddle_display_name = "{}, and {} others".format(
                 ', '.join(other_recipients[:2]), len(other_recipients) - 2)
             context.update({'huddle_display_name': huddle_display_name})
     elif (missed_messages[0]['message'].recipient.type == Recipient.PERSONAL):
@@ -388,7 +379,7 @@ def do_send_missedmessage_events_reply_in_zulip(user_profile: UserProfile,
                             m['trigger'] == 'wildcard_mentioned'})
         message = missed_messages[0]['message']
         stream = Stream.objects.only('id', 'name').get(id=message.recipient.type_id)
-        stream_header = "%s > %s" % (stream.name, message.topic_name())
+        stream_header = f"{stream.name} > {message.topic_name()}"
         context.update({
             'stream_header': stream_header,
         })
diff --git a/zerver/lib/email_validation.py b/zerver/lib/email_validation.py
index a50faf2d89..d67398fd98 100644
--- a/zerver/lib/email_validation.py
+++ b/zerver/lib/email_validation.py
@@ -112,7 +112,7 @@ def validate_email_is_valid(
     return None
 
 def email_reserved_for_system_bots_error(email: str) -> str:
-    return '%s is reserved for system bots' % (email,)
+    return f'{email} is reserved for system bots'
 
 def get_existing_user_errors(
     target_realm: Realm,
diff --git a/zerver/lib/error_notify.py b/zerver/lib/error_notify.py
index 1e29cda2cd..b416046f40 100644
--- a/zerver/lib/error_notify.py
+++ b/zerver/lib/error_notify.py
@@ -34,7 +34,7 @@ def user_info_str(report: Dict[str, Any]) -> str:
 def deployment_repr(report: Dict[str, Any]) -> str:
     deployment = 'Deployed code:\n'
     for field, val in report['deployment_data'].items():
-        deployment += '- %s: %s\n' % (field, val)
+        deployment += f'- {field}: {val}\n'
     return deployment
 
 def notify_browser_error(report: Dict[str, Any]) -> None:
@@ -44,7 +44,7 @@ def notify_browser_error(report: Dict[str, Any]) -> None:
     email_browser_error(report)
 
 def email_browser_error(report: Dict[str, Any]) -> None:
-    email_subject = "Browser error for %s" % (user_info_str(report),)
+    email_subject = f"Browser error for {user_info_str(report)}"
 
     body = ("User: %(user_full_name)s <%(user_email)s> on %(deployment)s\n\n"
             "Message:\n%(message)s\n\nStacktrace:\n%(stacktrace)s\n\n"
@@ -59,18 +59,18 @@ def email_browser_error(report: Dict[str, Any]) -> None:
     if more_info is not None:
         body += "\nAdditional information:"
         for (key, value) in more_info.items():
-            body += "\n  %s: %s" % (key, value)
+            body += f"\n  {key}: {value}"
 
-    body += "\n\nLog:\n%s" % (report['log'],)
+    body += "\n\nLog:\n{}".format(report['log'])
 
     mail_admins(email_subject, body)
 
 def zulip_browser_error(report: Dict[str, Any]) -> None:
-    email_subject = "JS error: %s" % (report['user_email'],)
+    email_subject = "JS error: {}".format(report['user_email'])
 
     user_info = user_info_str(report)
 
-    body = "User: %s\n" % (user_info,)
+    body = f"User: {user_info}\n"
     body += ("Message: %(message)s\n"
              % dict(report))
 
@@ -108,7 +108,7 @@ def zulip_server_error(report: Dict[str, Any]) -> None:
             val = report.get(field.lower())
             if field == "QUERY_STRING":
                 val = clean_data_from_query_parameters(str(val))
-            request_repr += "- %s: \"%s\"\n" % (field, val)
+            request_repr += f"- {field}: \"{val}\"\n"
         request_repr += "~~~~"
     else:
         request_repr = "Request info: none"
@@ -144,7 +144,7 @@ def email_server_error(report: Dict[str, Any]) -> None:
             val = report.get(field.lower())
             if field == "QUERY_STRING":
                 val = clean_data_from_query_parameters(str(val))
-            request_repr += "- %s: \"%s\"\n" % (field, val)
+            request_repr += f"- {field}: \"{val}\"\n"
     else:
         request_repr = "Request info: none\n"
 
diff --git a/zerver/lib/events.py b/zerver/lib/events.py
index 225e819863..e71a0595de 100644
--- a/zerver/lib/events.py
+++ b/zerver/lib/events.py
@@ -818,7 +818,7 @@ def apply_event(state: Dict[str, Any],
     elif event['type'] == 'has_zoom_token':
         state['has_zoom_token'] = event['value']
     else:
-        raise AssertionError("Unexpected event type %s" % (event['type'],))
+        raise AssertionError("Unexpected event type {}".format(event['type']))
 
 def do_events_register(user_profile: UserProfile, user_client: Client,
                        apply_markdown: bool = True,
diff --git a/zerver/lib/export.py b/zerver/lib/export.py
index b35b61e49f..163976bbe4 100644
--- a/zerver/lib/export.py
+++ b/zerver/lib/export.py
@@ -420,13 +420,13 @@ class Config:
                     using id_source.''')
             if self.id_source[0] != self.virtual_parent.table:
                 raise AssertionError('''
-                    Configuration error.  To populate %s, you
-                    want data from %s, but that differs from
-                    the table name of your virtual parent (%s),
+                    Configuration error.  To populate {}, you
+                    want data from {}, but that differs from
+                    the table name of your virtual parent ({}),
                     which suggests you many not have set up
                     the ordering correctly.  You may simply
                     need to assign a virtual_parent, or there
-                    may be deeper issues going on.''' % (
+                    may be deeper issues going on.'''.format(
                     self.table,
                     self.id_source[0],
                     self.virtual_parent.table))
@@ -466,7 +466,7 @@ def export_from_config(response: TableData, config: Config, seed_object: Optiona
         if config.custom_tables:
             for t in config.custom_tables:
                 if t not in response:
-                    raise AssertionError('Custom fetch failed to populate %s' % (t,))
+                    raise AssertionError(f'Custom fetch failed to populate {t}')
 
     elif config.concat_and_destroy:
         # When we concat_and_destroy, we are working with
@@ -1177,16 +1177,16 @@ def _check_key_metadata(email_gateway_bot: Optional[UserProfile],
     # Helper function for export_files_from_s3
     if 'realm_id' in key.metadata and key.metadata['realm_id'] != str(realm.id):
         if email_gateway_bot is None or key.metadata['user_profile_id'] != str(email_gateway_bot.id):
-            raise AssertionError("Key metadata problem: %s %s / %s" % (key.name, key.metadata, realm.id))
+            raise AssertionError(f"Key metadata problem: {key.name} {key.metadata} / {realm.id}")
         # Email gateway bot sends messages, potentially including attachments, cross-realm.
-        print("File uploaded by email gateway bot: %s / %s" % (key.key, key.metadata))
+        print(f"File uploaded by email gateway bot: {key.key} / {key.metadata}")
     elif processing_avatars:
         if 'user_profile_id' not in key.metadata:
-            raise AssertionError("Missing user_profile_id in key metadata: %s" % (key.metadata,))
+            raise AssertionError(f"Missing user_profile_id in key metadata: {key.metadata}")
         if int(key.metadata['user_profile_id']) not in user_ids:
-            raise AssertionError("Wrong user_profile_id in key metadata: %s" % (key.metadata,))
+            raise AssertionError(f"Wrong user_profile_id in key metadata: {key.metadata}")
     elif 'realm_id' not in key.metadata:
-        raise AssertionError("Missing realm_id in key metadata: %s" % (key.metadata,))
+        raise AssertionError(f"Missing realm_id in key metadata: {key.metadata}")
 
 def _get_exported_s3_record(
         bucket_name: str,
@@ -1233,11 +1233,11 @@ def _save_s3_object_to_file(key: ServiceResource, output_dir: str, processing_av
     else:
         fields = key.key.split('/')
         if len(fields) != 3:
-            raise AssertionError("Suspicious key with invalid format %s" % (key.key,))
+            raise AssertionError(f"Suspicious key with invalid format {key.key}")
         filename = os.path.join(output_dir, key.key)
 
     if "../" in filename:
-        raise AssertionError("Suspicious file with invalid format %s" % (filename,))
+        raise AssertionError(f"Suspicious file with invalid format {filename}")
 
     dirname = os.path.dirname(filename)
     if not os.path.exists(dirname):
@@ -1264,11 +1264,11 @@ def export_files_from_s3(realm: Realm, bucket_name: str, output_dir: Path,
             user_ids.add(user_profile.id)
 
     if processing_realm_icon_and_logo:
-        object_prefix = "%s/realm/" % (realm.id,)
+        object_prefix = f"{realm.id}/realm/"
     elif processing_emoji:
-        object_prefix = "%s/emoji/images/" % (realm.id,)
+        object_prefix = f"{realm.id}/emoji/images/"
     else:
-        object_prefix = "%s/" % (realm.id,)
+        object_prefix = f"{realm.id}/"
 
     if settings.EMAIL_GATEWAY_BOT is not None:
         email_gateway_bot: Optional[UserProfile] = get_system_bot(settings.EMAIL_GATEWAY_BOT)
@@ -1576,7 +1576,7 @@ def launch_user_message_subprocesses(threads: int, output_dir: Path,
     while pids:
         pid, status = os.wait()
         shard = pids.pop(pid)
-        print('Shard %s finished, status %s' % (shard, status))
+        print(f'Shard {shard} finished, status {status}')
 
 def do_export_user(user_profile: UserProfile, output_dir: Path) -> None:
     response: TableData = {}
@@ -1741,8 +1741,8 @@ def export_realm_wrapper(realm: Realm, output_dir: str,
     tarball_path = do_export_realm(realm=realm, output_dir=output_dir,
                                    threads=threads, public_only=public_only,
                                    consent_message_id=consent_message_id)
-    print("Finished exporting to %s" % (output_dir,))
-    print("Tarball written to %s" % (tarball_path,))
+    print(f"Finished exporting to {output_dir}")
+    print(f"Tarball written to {tarball_path}")
 
     if not upload:
         return None
@@ -1753,11 +1753,11 @@ def export_realm_wrapper(realm: Realm, output_dir: str,
     print("Uploading export tarball...")
     public_url = zerver.lib.upload.upload_backend.upload_export_tarball(realm, tarball_path)
     print()
-    print("Uploaded to %s" % (public_url,))
+    print(f"Uploaded to {public_url}")
 
     if delete_after_upload:
         os.remove(tarball_path)
-        print("Successfully deleted the tarball at %s" % (tarball_path,))
+        print(f"Successfully deleted the tarball at {tarball_path}")
     return public_url
 
 def get_realm_exports_serialized(user: UserProfile) -> List[Dict[str, Any]]:
diff --git a/zerver/lib/html_diff.py b/zerver/lib/html_diff.py
index 9f36c50891..16686a69e4 100644
--- a/zerver/lib/html_diff.py
+++ b/zerver/lib/html_diff.py
@@ -4,7 +4,7 @@ from lxml.html.diff import htmldiff
 from typing import Optional
 
 def highlight_with_class(text: str, klass: str) -> str:
-    return '<span class="%s">%s</span>' % (klass, text)
+    return f'<span class="{klass}">{text}</span>'
 
 def highlight_html_differences(s1: str, s2: str, msg_id: Optional[int]=None) -> str:
     retval = htmldiff(s1, s2)
diff --git a/zerver/lib/import_realm.py b/zerver/lib/import_realm.py
index 8da3306877..6348aea44c 100644
--- a/zerver/lib/import_realm.py
+++ b/zerver/lib/import_realm.py
@@ -101,10 +101,10 @@ path_maps: Dict[str, Dict[str, str]] = {
 def update_id_map(table: TableName, old_id: int, new_id: int) -> None:
     if table not in ID_MAP:
         raise Exception('''
-            Table %s is not initialized in ID_MAP, which could
+            Table {} is not initialized in ID_MAP, which could
             mean that we have not thought through circular
             dependencies.
-            ''' % (table,))
+            '''.format(table))
     ID_MAP[table][old_id] = new_id
 
 def fix_datetime_fields(data: TableData, table: TableName) -> None:
diff --git a/zerver/lib/logging_util.py b/zerver/lib/logging_util.py
index 40d621857c..23ddb43e21 100644
--- a/zerver/lib/logging_util.py
+++ b/zerver/lib/logging_util.py
@@ -70,7 +70,7 @@ class _RateLimitFilter:
         try:
             # Track duplicate errors
             duplicate = False
-            rate = getattr(settings, '%s_LIMIT' % (self.__class__.__name__.upper(),),
+            rate = getattr(settings, f'{self.__class__.__name__.upper()}_LIMIT',
                            600)  # seconds
 
             if rate > 0:
diff --git a/zerver/lib/management.py b/zerver/lib/management.py
index 7a7001b094..f7c90fa9f5 100644
--- a/zerver/lib/management.py
+++ b/zerver/lib/management.py
@@ -28,7 +28,7 @@ def check_config() -> None:
         except AttributeError:
             pass
 
-        raise CommandError("Error: You must set %s in /etc/zulip/settings.py." % (setting_name,))
+        raise CommandError(f"Error: You must set {setting_name} in /etc/zulip/settings.py.")
 
 def sleep_forever() -> None:
     while True:  # nocoverage
@@ -127,7 +127,7 @@ You can use the command list_realms to find ID of the realms in this server."""
                 return UserProfile.objects.select_related().get(
                     delivery_email__iexact=email.strip(), realm=realm)
             except UserProfile.DoesNotExist:
-                raise CommandError("The realm '%s' does not contain a user with email '%s'" % (realm, email))
+                raise CommandError(f"The realm '{realm}' does not contain a user with email '{email}'")
 
         # Realm is None in the remaining code path.  Here, we
         # optimistically try to see if there is exactly one user with
@@ -139,7 +139,7 @@ You can use the command list_realms to find ID of the realms in this server."""
                                "(in different realms); please pass `--realm` "
                                "to specify which one to modify.")
         except UserProfile.DoesNotExist:
-            raise CommandError("This Zulip server does not contain a user with email '%s'" % (email,))
+            raise CommandError(f"This Zulip server does not contain a user with email '{email}'")
 
     def get_client(self) -> Client:
         """Returns a Zulip Client object to be used for things done in management commands"""
diff --git a/zerver/lib/message.py b/zerver/lib/message.py
index 3c2eff8552..b944684710 100644
--- a/zerver/lib/message.py
+++ b/zerver/lib/message.py
@@ -519,7 +519,7 @@ class MessageDict:
                 elif recip['email'] > display_recipient[0]['email']:
                     display_recipient = [display_recipient[0], recip]
         else:
-            raise AssertionError("Invalid recipient type %s" % (recipient_type,))
+            raise AssertionError(f"Invalid recipient type {recipient_type}")
 
         obj['display_recipient'] = display_recipient
         obj['type'] = display_type
@@ -1019,7 +1019,7 @@ def apply_unread_message_event(user_profile: UserProfile,
         else:
             message_type = 'huddle'
     else:
-        raise AssertionError("Invalid message type %s" % (message['type'],))
+        raise AssertionError("Invalid message type {}".format(message['type']))
 
     sender_id = message['sender_id']
 
diff --git a/zerver/lib/migrate.py b/zerver/lib/migrate.py
index be0830a888..3fa2bd3167 100644
--- a/zerver/lib/migrate.py
+++ b/zerver/lib/migrate.py
@@ -28,11 +28,11 @@ def do_batch_update(cursor: CursorObj,
     if min_id is None:
         return
 
-    print("\n    Range of rows to update: [%s, %s]" % (min_id, max_id))
+    print(f"\n    Range of rows to update: [{min_id}, {max_id}]")
     while min_id <= max_id:
         lower = min_id
         upper = min_id + batch_size
-        print('    Updating range [%s,%s)' % (lower, upper))
+        print(f'    Updating range [{lower},{upper})')
         cursor.execute(stmt, [lower, upper])
 
         min_id = upper
diff --git a/zerver/lib/mobile_auth_otp.py b/zerver/lib/mobile_auth_otp.py
index f04a54ad57..83df1fef71 100644
--- a/zerver/lib/mobile_auth_otp.py
+++ b/zerver/lib/mobile_auth_otp.py
@@ -15,7 +15,7 @@ def xor_hex_strings(bytes_a: str, bytes_b: str) -> str:
     """Given two hex strings of equal length, return a hex string with
     the bitwise xor of the two hex strings."""
     assert len(bytes_a) == len(bytes_b)
-    return ''.join(["%x" % (int(x, 16) ^ int(y, 16),)
+    return ''.join([f"{int(x, 16) ^ int(y, 16):x}"
                     for x, y in zip(bytes_a, bytes_b)])
 
 def ascii_to_hex(input_string: str) -> str:
diff --git a/zerver/lib/outgoing_webhook.py b/zerver/lib/outgoing_webhook.py
index 099c3457d9..cc5c56b173 100644
--- a/zerver/lib/outgoing_webhook.py
+++ b/zerver/lib/outgoing_webhook.py
@@ -223,11 +223,11 @@ def notify_bot_owner(event: Dict[str, Any],
     bot_id = event['user_profile_id']
     bot_owner = get_user_profile_by_id(bot_id).bot_owner
 
-    notification_message = "[A message](%s) triggered an outgoing webhook." % (message_url,)
+    notification_message = f"[A message]({message_url}) triggered an outgoing webhook."
     if failure_message:
         notification_message += "\n" + failure_message
     if status_code:
-        notification_message += "\nThe webhook got a response with status code *%s*." % (status_code,)
+        notification_message += f"\nThe webhook got a response with status code *{status_code}*."
     if response_content:
         notification_message += "\nThe response contains the following payload:\n" \
                                 "```\n%s\n```" % (str(response_content),)
@@ -326,6 +326,6 @@ def do_rest_call(base_url: str,
         response_message = ("An exception of type *%s* occurred for message `%s`! "
                             "See the Zulip server logs for more information." % (
                                 type(e).__name__, event["command"],))
-        logging.exception("Outhook trigger failed:\n %s" % (e,))
+        logging.exception(f"Outhook trigger failed:\n {e}")
         fail_with_message(event, response_message)
         notify_bot_owner(event, exception=e)
diff --git a/zerver/lib/push_notifications.py b/zerver/lib/push_notifications.py
index a6aa895d3d..5abbdd370a 100644
--- a/zerver/lib/push_notifications.py
+++ b/zerver/lib/push_notifications.py
@@ -468,14 +468,14 @@ def get_gcm_alert(message: Message) -> str:
     """
     sender_str = message.sender.full_name
     if message.recipient.type == Recipient.HUDDLE and message.trigger == 'private_message':
-        return "New private group message from %s" % (sender_str,)
+        return f"New private group message from {sender_str}"
     elif message.recipient.type == Recipient.PERSONAL and message.trigger == 'private_message':
-        return "New private message from %s" % (sender_str,)
+        return f"New private message from {sender_str}"
     elif message.is_stream_message() and (message.trigger == 'mentioned' or
                                           message.trigger == 'wildcard_mentioned'):
-        return "New mention from %s" % (sender_str,)
+        return f"New mention from {sender_str}"
     else:  # message.is_stream_message() and message.trigger == 'stream_push_notify'
-        return "New stream message from %s in %s" % (sender_str, get_display_recipient(message.recipient),)
+        return f"New stream message from {sender_str} in {get_display_recipient(message.recipient)}"
 
 def get_mobile_push_content(rendered_content: str) -> str:
     def get_text(elem: lxml.html.HtmlElement) -> str:
@@ -586,7 +586,7 @@ def get_apns_alert_title(message: Message) -> str:
         assert isinstance(recipients, list)
         return ', '.join(sorted(r['full_name'] for r in recipients))
     elif message.is_stream_message():
-        return "#%s > %s" % (get_display_recipient(message.recipient), message.topic_name(),)
+        return f"#{get_display_recipient(message.recipient)} > {message.topic_name()}"
     # For personal PMs, we just show the sender name.
     return message.sender.full_name
 
diff --git a/zerver/lib/queue.py b/zerver/lib/queue.py
index e827320ac0..176a06ac02 100644
--- a/zerver/lib/queue.py
+++ b/zerver/lib/queue.py
@@ -38,7 +38,7 @@ class SimpleQueueClient:
         start = time.time()
         self.connection = pika.BlockingConnection(self._get_parameters())
         self.channel    = self.connection.channel()
-        self.log.info('SimpleQueueClient connected (connecting took %.3fs)' % (time.time() - start,))
+        self.log.info(f'SimpleQueueClient connected (connecting took {time.time() - start:.3f}s)')
 
     def _reconnect(self) -> None:
         self.connection = None
@@ -76,10 +76,10 @@ class SimpleQueueClient:
                                          credentials=credentials)
 
     def _generate_ctag(self, queue_name: str) -> str:
-        return "%s_%s" % (queue_name, str(random.getrandbits(16)))
+        return f"{queue_name}_{str(random.getrandbits(16))}"
 
     def _reconnect_consumer_callback(self, queue: str, consumer: Consumer) -> None:
-        self.log.info("Queue reconnecting saved consumer %s to queue %s" % (consumer, queue))
+        self.log.info(f"Queue reconnecting saved consumer {consumer} to queue {queue}")
         self.ensure_queue(queue, lambda: self.channel.basic_consume(queue,
                                                                     consumer,
                                                                     consumer_tag=self._generate_ctag(queue)))
@@ -115,7 +115,7 @@ class SimpleQueueClient:
                 properties=pika.BasicProperties(delivery_mode=2),
                 body=body)
 
-            statsd.incr("rabbitmq.publish.%s" % (queue_name,))
+            statsd.incr(f"rabbitmq.publish.{queue_name}")
 
         self.ensure_queue(queue_name, do_publish)
 
diff --git a/zerver/lib/rate_limiter.py b/zerver/lib/rate_limiter.py
index e980e46958..447f67868b 100644
--- a/zerver/lib/rate_limiter.py
+++ b/zerver/lib/rate_limiter.py
@@ -286,7 +286,7 @@ class TornadoInMemoryRateLimiterBackend(RateLimiterBackend):
             ratelimited, time_till_free = cls.need_to_limit(entity_key, time_window, max_count)
 
             if ratelimited:
-                statsd.incr("ratelimiter.limited.%s" % (entity_key,))
+                statsd.incr(f"ratelimiter.limited.{entity_key}")
                 break
 
         return ratelimited, time_till_free
@@ -450,7 +450,7 @@ class RedisRateLimiterBackend(RateLimiterBackend):
         ratelimited, time = cls.is_ratelimited(entity_key, rules)
 
         if ratelimited:
-            statsd.incr("ratelimiter.limited.%s" % (entity_key,))
+            statsd.incr(f"ratelimiter.limited.{entity_key}")
 
         else:
             try:
diff --git a/zerver/lib/realm_icon.py b/zerver/lib/realm_icon.py
index b925247e4d..81a9eeb1d1 100644
--- a/zerver/lib/realm_icon.py
+++ b/zerver/lib/realm_icon.py
@@ -12,6 +12,6 @@ def get_realm_icon_url(realm: Realm) -> str:
         return upload_backend.get_realm_icon_url(realm.id, realm.icon_version)
     elif settings.ENABLE_GRAVATAR:
         hash_key = gravatar_hash(realm.string_id)
-        return "https://secure.gravatar.com/avatar/%s?d=identicon" % (hash_key,)
+        return f"https://secure.gravatar.com/avatar/{hash_key}?d=identicon"
     else:
         return settings.DEFAULT_AVATAR_URI+'?version=0'
diff --git a/zerver/lib/redis_utils.py b/zerver/lib/redis_utils.py
index e5d6dc718c..27adf96e1d 100644
--- a/zerver/lib/redis_utils.py
+++ b/zerver/lib/redis_utils.py
@@ -63,4 +63,4 @@ def validate_key_fits_format(key: str, key_format: str) -> None:
     regex = key_format.format(token=r"[a-zA-Z0-9]+")
 
     if not re.fullmatch(regex, key):
-        raise ZulipRedisKeyOfWrongFormatError("%s does not match format %s" % (key, key_format))
+        raise ZulipRedisKeyOfWrongFormatError(f"{key} does not match format {key_format}")
diff --git a/zerver/lib/remote_server.py b/zerver/lib/remote_server.py
index 8ab76f29c9..892c249b79 100644
--- a/zerver/lib/remote_server.py
+++ b/zerver/lib/remote_server.py
@@ -45,7 +45,7 @@ def send_to_push_bouncer(method: str,
     api_auth = requests.auth.HTTPBasicAuth(settings.ZULIP_ORG_ID,
                                            settings.ZULIP_ORG_KEY)
 
-    headers = {"User-agent": "ZulipServer/%s" % (ZULIP_VERSION,)}
+    headers = {"User-agent": f"ZulipServer/{ZULIP_VERSION}"}
     if extra_headers is not None:
         headers.update(extra_headers)
 
@@ -88,7 +88,7 @@ def send_to_push_bouncer(method: str,
         # this version of Zulip, so we throw an exception that will
         # email the server admins.
         raise PushNotificationBouncerException(
-            "Push notification bouncer returned unexpected status code %s" % (res.status_code,))
+            f"Push notification bouncer returned unexpected status code {res.status_code}")
 
     # If we don't throw an exception, it's a successful bounce!
     return ujson.loads(res.content)
diff --git a/zerver/lib/response.py b/zerver/lib/response.py
index c499bb81d7..52e1576401 100644
--- a/zerver/lib/response.py
+++ b/zerver/lib/response.py
@@ -11,9 +11,9 @@ class HttpResponseUnauthorized(HttpResponse):
     def __init__(self, realm: str, www_authenticate: Optional[str]=None) -> None:
         HttpResponse.__init__(self)
         if www_authenticate is None:
-            self["WWW-Authenticate"] = 'Basic realm="%s"' % (realm,)
+            self["WWW-Authenticate"] = f'Basic realm="{realm}"'
         elif www_authenticate == "session":
-            self["WWW-Authenticate"] = 'Session realm="%s"' % (realm,)
+            self["WWW-Authenticate"] = f'Session realm="{realm}"'
         else:
             raise AssertionError("Invalid www_authenticate value!")
 
diff --git a/zerver/lib/rest.py b/zerver/lib/rest.py
index 37b36264ab..3954b4aa05 100644
--- a/zerver/lib/rest.py
+++ b/zerver/lib/rest.py
@@ -144,7 +144,7 @@ def rest_dispatch(request: HttpRequest, **kwargs: Any) -> HttpResponse:
             # browser, send the user to the login page
             if 'text/html' in request.META.get('HTTP_ACCEPT', ''):
                 # TODO: It seems like the `?next=` part is unlikely to be helpful
-                return HttpResponseRedirect('%s?next=%s' % (settings.HOME_NOT_LOGGED_IN, request.path))
+                return HttpResponseRedirect(f'{settings.HOME_NOT_LOGGED_IN}?next={request.path}')
             # Ask for basic auth (email:apiKey)
             elif request.path.startswith("/api"):
                 return json_unauthorized()
diff --git a/zerver/lib/send_email.py b/zerver/lib/send_email.py
index e6cf45fb39..3642935efc 100644
--- a/zerver/lib/send_email.py
+++ b/zerver/lib/send_email.py
@@ -265,12 +265,12 @@ def send_custom_email(users: List[UserProfile], options: Dict[str, Any]) -> None
         parsed_email_template = Parser(policy=default).parsestr(text)
         email_template_hash = hashlib.sha256(text.encode('utf-8')).hexdigest()[0:32]
 
-    email_filename = "custom/custom_email_%s.source.html" % (email_template_hash,)
-    email_id = "zerver/emails/custom/custom_email_%s" % (email_template_hash,)
+    email_filename = f"custom/custom_email_{email_template_hash}.source.html"
+    email_id = f"zerver/emails/custom/custom_email_{email_template_hash}"
     markdown_email_base_template_path = "templates/zerver/emails/custom_email_base.pre.html"
-    html_source_template_path = "templates/%s.source.html" % (email_id,)
-    plain_text_template_path = "templates/%s.txt" % (email_id,)
-    subject_path = "templates/%s.subject.txt" % (email_id,)
+    html_source_template_path = f"templates/{email_id}.source.html"
+    plain_text_template_path = f"templates/{email_id}.txt"
+    subject_path = f"templates/{email_id}.subject.txt"
     os.makedirs(os.path.dirname(html_source_template_path), exist_ok=True)
 
     # First, we render the markdown input file just like our
diff --git a/zerver/lib/soft_deactivation.py b/zerver/lib/soft_deactivation.py
index b6611917e9..ef94fffb75 100644
--- a/zerver/lib/soft_deactivation.py
+++ b/zerver/lib/soft_deactivation.py
@@ -74,7 +74,7 @@ def filter_by_subscription_history(user_profile: UserProfile,
                     if stream_messages[-1]['id'] <= log_entry.event_last_message_id:
                         stream_messages = []
             else:
-                raise AssertionError('%s is not a Subscription Event.' % (log_entry.event_type,))
+                raise AssertionError(f'{log_entry.event_type} is not a Subscription Event.')
 
         if len(stream_messages) > 0:
             # We do this check for last event since if the last subscription
diff --git a/zerver/lib/subdomains.py b/zerver/lib/subdomains.py
index 476f0c8523..77800dc1aa 100644
--- a/zerver/lib/subdomains.py
+++ b/zerver/lib/subdomains.py
@@ -23,7 +23,7 @@ def get_subdomain(request: HttpRequest) -> str:
     return get_subdomain_from_hostname(host)
 
 def get_subdomain_from_hostname(host: str) -> str:
-    m = re.search(r'\.%s(:\d+)?$' % (settings.EXTERNAL_HOST,),
+    m = re.search(fr'\.{settings.EXTERNAL_HOST}(:\d+)?$',
                   host)
     if m:
         subdomain = host[:m.start()]
@@ -32,7 +32,7 @@ def get_subdomain_from_hostname(host: str) -> str:
         return subdomain
 
     for subdomain, realm_host in settings.REALM_HOSTS.items():
-        if re.search(r'^%s(:\d+)?$' % (realm_host,),
+        if re.search(fr'^{realm_host}(:\d+)?$',
                      host):
             return subdomain
 
diff --git a/zerver/lib/test_classes.py b/zerver/lib/test_classes.py
index f8e4d28bda..74d59b62a4 100644
--- a/zerver/lib/test_classes.py
+++ b/zerver/lib/test_classes.py
@@ -487,7 +487,7 @@ class ZulipTestCase(TestCase):
         """
         identifier: Can be an email or a remote server uuid.
         """
-        credentials = "%s:%s" % (identifier, api_key)
+        credentials = f"{identifier}:{api_key}"
         return 'Basic ' + base64.b64encode(credentials.encode('utf-8')).decode('utf-8')
 
     def uuid_get(self, identifier: str, *args: Any, **kwargs: Any) -> HttpResponse:
@@ -629,7 +629,7 @@ class ZulipTestCase(TestCase):
             print('ITEMS:\n')
             for item in items:
                 print(item)
-            print("\nexpected length: %s\nactual length: %s" % (count, actual_count))
+            print(f"\nexpected length: {count}\nactual length: {actual_count}")
             raise AssertionError('List is unexpected size!')
 
     def assert_json_error_contains(self, result: HttpResponse, msg_substring: str,
@@ -663,14 +663,14 @@ class ZulipTestCase(TestCase):
     def webhook_fixture_data(self, type: str, action: str, file_type: str='json') -> str:
         fn = os.path.join(
             os.path.dirname(__file__),
-            "../webhooks/%s/fixtures/%s.%s" % (type, action, file_type)
+            f"../webhooks/{type}/fixtures/{action}.{file_type}"
         )
         return open(fn).read()
 
     def fixture_file_name(self, file_name: str, type: str='') -> str:
         return os.path.join(
             os.path.dirname(__file__),
-            "../tests/fixtures/%s/%s" % (type, file_name)
+            f"../tests/fixtures/{type}/{file_name}"
         )
 
     def fixture_data(self, file_name: str, type: str='') -> str:
@@ -695,10 +695,10 @@ class ZulipTestCase(TestCase):
             )
         except IntegrityError:  # nocoverage -- this is for bugs in the tests
             raise Exception('''
-                %s already exists
+                {} already exists
 
                 Please call make_stream with a stream name
-                that is not already in use.''' % (stream_name,))
+                that is not already in use.'''.format(stream_name))
 
         recipient = Recipient.objects.create(type_id=stream.id, type=Recipient.STREAM)
         stream.recipient = recipient
diff --git a/zerver/lib/test_fixtures.py b/zerver/lib/test_fixtures.py
index 192ed5aad2..3327ff2955 100644
--- a/zerver/lib/test_fixtures.py
+++ b/zerver/lib/test_fixtures.py
@@ -357,7 +357,7 @@ def destroy_leaked_test_databases(expiry_time: int = 60 * 60) -> int:
     if not databases_to_drop:
         return 0
 
-    commands = "\n".join("DROP DATABASE IF EXISTS %s;" % (db,) for db in databases_to_drop)
+    commands = "\n".join(f"DROP DATABASE IF EXISTS {db};" for db in databases_to_drop)
     p = subprocess.Popen(["psql", "-q", "-v", "ON_ERROR_STOP=1", "-h", "localhost",
                           "postgres", "zulip_test"],
                          stdin=subprocess.PIPE)
diff --git a/zerver/lib/test_helpers.py b/zerver/lib/test_helpers.py
index 0b041f7b47..75602a3ae9 100644
--- a/zerver/lib/test_helpers.py
+++ b/zerver/lib/test_helpers.py
@@ -161,7 +161,7 @@ def queries_captured(include_savepoints: Optional[bool]=False) -> Generator[
             if include_savepoints or not isinstance(sql, str) or 'SAVEPOINT' not in sql:
                 queries.append({
                     'sql': self.mogrify(sql, params).decode('utf-8'),
-                    'time': "%.3f" % (duration,),
+                    'time': f"{duration:.3f}",
                 })
 
     old_execute = TimeTrackingCursor.execute
@@ -437,13 +437,13 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
                     print(call)
 
         if full_suite:
-            print('INFO: URL coverage report is in %s' % (fn,))
+            print(f'INFO: URL coverage report is in {fn}')
             print('INFO: Try running: ./tools/create-test-api-docs')
 
         if full_suite and len(untested_patterns):  # nocoverage -- test suite error handling
             print("\nERROR: Some URLs are untested!  Here's the list of untested URLs:")
             for untested_pattern in sorted(untested_patterns):
-                print("   %s" % (untested_pattern,))
+                print(f"   {untested_pattern}")
             sys.exit(1)
 
 def load_subdomain_token(response: HttpResponse) -> ExternalAuthDataDict:
diff --git a/zerver/lib/test_runner.py b/zerver/lib/test_runner.py
index 2c10c05113..3b5b5a3c69 100644
--- a/zerver/lib/test_runner.py
+++ b/zerver/lib/test_runner.py
@@ -74,7 +74,7 @@ def full_test_name(test: TestCase) -> str:
     test_module = test.__module__
     test_class = test.__class__.__name__
     test_method = test._testMethodName
-    return '%s.%s.%s' % (test_module, test_class, test_method)
+    return f'{test_module}.{test_class}.{test_method}'
 
 def get_test_method(test: TestCase) -> Callable[[], None]:
     return getattr(test, test._testMethodName)
@@ -91,14 +91,14 @@ def report_slow_tests() -> None:
     for delay, test_name, slowness_reason in timings[:15]:
         if not slowness_reason:
             slowness_reason = 'UNKNOWN WHY SLOW, please investigate'
-        print(' %0.3f %s\n       %s\n' % (delay, test_name, slowness_reason))
+        print(f' {delay:0.3f} {test_name}\n       {slowness_reason}\n')
 
     print('...')
     for delay, test_name, slowness_reason in timings[100:]:
         if slowness_reason:
-            print(' %.3f %s is not that slow' % (delay, test_name))
+            print(f' {delay:.3f} {test_name} is not that slow')
             print('      consider removing @slow decorator')
-            print('      This may no longer be true: %s' % (slowness_reason,))
+            print(f'      This may no longer be true: {slowness_reason}')
 
 def enforce_timely_test_completion(test_method: Callable[..., ReturnT], test_name: str,
                                    delay: float, result: unittest.TestResult) -> None:
@@ -110,7 +110,7 @@ def enforce_timely_test_completion(test_method: Callable[..., ReturnT], test_nam
     assert isinstance(result, TextTestResult) or isinstance(result, RemoteTestResult)
 
     if delay > max_delay:
-        msg = '** Test is TOO slow: %s (%.3f s)\n' % (test_name, delay)
+        msg = f'** Test is TOO slow: {test_name} ({delay:.3f} s)\n'
         result.addInfo(test_method, msg)
 
 def fast_tests_only() -> bool:
diff --git a/zerver/lib/thumbnail.py b/zerver/lib/thumbnail.py
index 9ee9517430..2cc19a7da8 100644
--- a/zerver/lib/thumbnail.py
+++ b/zerver/lib/thumbnail.py
@@ -46,7 +46,7 @@ def generate_thumbnail_url(path: str,
 
     source_type = get_source_type(path)
     safe_url = base64.urlsafe_b64encode(path.encode()).decode('utf-8')
-    image_url = '%s/source_type/%s' % (safe_url, source_type)
+    image_url = f'{safe_url}/source_type/{source_type}'
     width, height = map(int, size.split('x'))
     crypto = CryptoURL(key=settings.THUMBOR_KEY)
 
diff --git a/zerver/lib/timestamp.py b/zerver/lib/timestamp.py
index 7f4f47d64f..d344bef6f5 100644
--- a/zerver/lib/timestamp.py
+++ b/zerver/lib/timestamp.py
@@ -6,7 +6,7 @@ class TimezoneNotUTCException(Exception):
 
 def verify_UTC(dt: datetime.datetime) -> None:
     if dt.tzinfo is None or dt.tzinfo.utcoffset(dt) != datetime.timezone.utc.utcoffset(dt):
-        raise TimezoneNotUTCException("Datetime %s does not have a UTC timezone." % (dt,))
+        raise TimezoneNotUTCException(f"Datetime {dt} does not have a UTC timezone.")
 
 def convert_to_UTC(dt: datetime.datetime) -> datetime.datetime:
     if dt.tzinfo is None:
diff --git a/zerver/lib/type_debug.py b/zerver/lib/type_debug.py
index 4aa6f571cd..35a07fbc45 100644
--- a/zerver/lib/type_debug.py
+++ b/zerver/lib/type_debug.py
@@ -15,14 +15,14 @@ def get_mapping_type_str(x: Mapping[Any, Any]) -> str:
     value_type = get_type_str(x[key])
     if container_type == 'dict':
         if len(x) == 1:
-            return '{%s: %s}' % (key_type, value_type)
+            return f'{{{key_type}: {value_type}}}'
         else:
-            return '{%s: %s, ...}' % (key_type, value_type)
+            return f'{{{key_type}: {value_type}, ...}}'
     else:
         if len(x) == 1:
-            return '%s([(%s, %s)])' % (container_type, key_type, value_type)
+            return f'{container_type}([({key_type}, {value_type})])'
         else:
-            return '%s([(%s, %s), ...])' % (container_type, key_type, value_type)
+            return f'{container_type}([({key_type}, {value_type}), ...])'
 
 def get_sequence_type_str(x: Sequence[Any]) -> str:
     container_type = type(x).__name__
@@ -39,9 +39,9 @@ def get_sequence_type_str(x: Sequence[Any]) -> str:
             return '[' + elem_type + ', ...]'
     else:
         if len(x) == 1:
-            return '%s([%s])' % (container_type, elem_type)
+            return f'{container_type}([{elem_type}])'
         else:
-            return '%s([%s, ...])' % (container_type, elem_type)
+            return f'{container_type}([{elem_type}, ...])'
 
 expansion_blacklist = [str, bytes]
 
@@ -72,9 +72,9 @@ def print_types_to(file_obj: IO[str]) -> Callable[[FuncT], FuncT]:
             arg_types = [get_type_str(arg) for arg in args]
             kwarg_types = [key + "=" + get_type_str(value) for key, value in kwargs.items()]
             ret_val = func(*args, **kwargs)
-            output = "%s(%s) -> %s" % (func.__name__,
-                                       ", ".join(arg_types + kwarg_types),
-                                       get_type_str(ret_val))
+            output = "{}({}) -> {}".format(func.__name__,
+                                           ", ".join(arg_types + kwarg_types),
+                                           get_type_str(ret_val))
             print(output, file=file_obj)
             return ret_val
         return wrapper  # type: ignore[return-value] # https://github.com/python/mypy/issues/1927
diff --git a/zerver/lib/upload.py b/zerver/lib/upload.py
index a22751b73e..030db29994 100644
--- a/zerver/lib/upload.py
+++ b/zerver/lib/upload.py
@@ -378,7 +378,7 @@ class S3UploadBackend(ZulipUploadBackend):
             random_name(18),
             sanitize_name(uploaded_file_name)
         ])
-        url = "/user_uploads/%s" % (s3_file_name,)
+        url = f"/user_uploads/{s3_file_name}"
 
         upload_image_to_s3(
             bucket_name,
@@ -469,12 +469,12 @@ class S3UploadBackend(ZulipUploadBackend):
         bucket = settings.S3_AVATAR_BUCKET
         medium_suffix = "-medium.png" if medium else ""
         # ?x=x allows templates to append additional parameters with &s
-        return "https://%s.s3.amazonaws.com/%s%s?x=x" % (bucket, hash_key, medium_suffix)
+        return f"https://{bucket}.s3.amazonaws.com/{hash_key}{medium_suffix}?x=x"
 
     def get_export_tarball_url(self, realm: Realm, export_path: str) -> str:
         bucket = settings.S3_AVATAR_BUCKET
         # export_path has a leading /
-        return "https://%s.s3.amazonaws.com%s" % (bucket, export_path)
+        return f"https://{bucket}.s3.amazonaws.com{export_path}"
 
     def realm_avatar_and_logo_path(self, realm: Realm) -> str:
         return os.path.join(str(realm.id), 'realm')
@@ -507,8 +507,7 @@ class S3UploadBackend(ZulipUploadBackend):
     def get_realm_icon_url(self, realm_id: int, version: int) -> str:
         bucket = settings.S3_AVATAR_BUCKET
         # ?x=x allows templates to append additional parameters with &s
-        return "https://%s.s3.amazonaws.com/%s/realm/icon.png?version=%s" % (
-            bucket, realm_id, version)
+        return f"https://{bucket}.s3.amazonaws.com/{realm_id}/realm/icon.png?version={version}"
 
     def upload_realm_logo_image(self, logo_file: File, user_profile: UserProfile,
                                 night: bool) -> None:
@@ -547,8 +546,7 @@ class S3UploadBackend(ZulipUploadBackend):
             file_name = 'logo.png'
         else:
             file_name = 'night_logo.png'
-        return "https://%s.s3.amazonaws.com/%s/realm/%s?version=%s" % (
-            bucket, realm_id, file_name, version)
+        return f"https://{bucket}.s3.amazonaws.com/{realm_id}/realm/{file_name}?version={version}"
 
     def ensure_medium_avatar_image(self, user_profile: UserProfile) -> None:
         file_path = user_avatar_path(user_profile)
@@ -618,7 +616,7 @@ class S3UploadBackend(ZulipUploadBackend):
         bucket = settings.S3_AVATAR_BUCKET
         emoji_path = RealmEmoji.PATH_ID_TEMPLATE.format(realm_id=realm_id,
                                                         emoji_file_name=emoji_file_name)
-        return "https://%s.s3.amazonaws.com/%s" % (bucket, emoji_path)
+        return f"https://{bucket}.s3.amazonaws.com/{emoji_path}"
 
     def upload_export_tarball(self, realm: Optional[Realm], tarball_path: str) -> str:
         def percent_callback(bytes_transferred: Any) -> None:
@@ -747,7 +745,7 @@ class LocalUploadBackend(ZulipUploadBackend):
     def get_avatar_url(self, hash_key: str, medium: bool=False) -> str:
         # ?x=x allows templates to append additional parameters with &s
         medium_suffix = "-medium" if medium else ""
-        return "/user_avatars/%s%s.png?x=x" % (hash_key, medium_suffix)
+        return f"/user_avatars/{hash_key}{medium_suffix}.png?x=x"
 
     def copy_avatar(self, source_profile: UserProfile, target_profile: UserProfile) -> None:
         source_file_path = user_avatar_path(source_profile)
@@ -772,7 +770,7 @@ class LocalUploadBackend(ZulipUploadBackend):
 
     def get_realm_icon_url(self, realm_id: int, version: int) -> str:
         # ?x=x allows templates to append additional parameters with &s
-        return "/user_avatars/%s/realm/icon.png?version=%s" % (realm_id, version)
+        return f"/user_avatars/{realm_id}/realm/icon.png?version={version}"
 
     def upload_realm_logo_image(self, logo_file: File, user_profile: UserProfile,
                                 night: bool) -> None:
@@ -798,7 +796,7 @@ class LocalUploadBackend(ZulipUploadBackend):
             file_name = 'night_logo.png'
         else:
             file_name = 'logo.png'
-        return "/user_avatars/%s/realm/%s?version=%s" % (realm_id, file_name, version)
+        return f"/user_avatars/{realm_id}/realm/{file_name}?version={version}"
 
     def ensure_medium_avatar_image(self, user_profile: UserProfile) -> None:
         file_path = user_avatar_path(user_profile)
diff --git a/zerver/lib/url_encoding.py b/zerver/lib/url_encoding.py
index a223f8dc62..a5d59d031e 100644
--- a/zerver/lib/url_encoding.py
+++ b/zerver/lib/url_encoding.py
@@ -17,25 +17,23 @@ def encode_stream(stream_id: int, stream_name: str) -> str:
     return str(stream_id) + '-' + hash_util_encode(stream_name)
 
 def personal_narrow_url(realm: Realm, sender: UserProfile) -> str:
-    base_url = "%s/#narrow/pm-with/" % (realm.uri,)
+    base_url = f"{realm.uri}/#narrow/pm-with/"
     email_user = sender.email.split('@')[0].lower()
     pm_slug = str(sender.id) + '-' + hash_util_encode(email_user)
     return base_url + pm_slug
 
 def huddle_narrow_url(realm: Realm, other_user_ids: List[int]) -> str:
     pm_slug = ','.join(str(user_id) for user_id in sorted(other_user_ids)) + '-group'
-    base_url = "%s/#narrow/pm-with/" % (realm.uri,)
+    base_url = f"{realm.uri}/#narrow/pm-with/"
     return base_url + pm_slug
 
 def stream_narrow_url(realm: Realm, stream: Stream) -> str:
-    base_url = "%s/#narrow/stream/" % (realm.uri,)
+    base_url = f"{realm.uri}/#narrow/stream/"
     return base_url + encode_stream(stream.id, stream.name)
 
 def topic_narrow_url(realm: Realm, stream: Stream, topic: str) -> str:
-    base_url = "%s/#narrow/stream/" % (realm.uri,)
-    return "%s%s/topic/%s" % (base_url,
-                              encode_stream(stream.id, stream.name),
-                              hash_util_encode(topic))
+    base_url = f"{realm.uri}/#narrow/stream/"
+    return f"{base_url}{encode_stream(stream.id, stream.name)}/topic/{hash_util_encode(topic)}"
 
 def near_message_url(realm: Realm,
                      message: Dict[str, Any]) -> str:
diff --git a/zerver/lib/utils.py b/zerver/lib/utils.py
index 97c1e5ac03..4b1d35ef8e 100644
--- a/zerver/lib/utils.py
+++ b/zerver/lib/utils.py
@@ -36,9 +36,9 @@ class StatsDWrapper:
         """Set a gauge value."""
         from django_statsd.clients import statsd
         if delta:
-            value_str = '%+g|g' % (value,)
+            value_str = f'{value:+g}|g'
         else:
-            value_str = '%g|g' % (value,)
+            value_str = f'{value:g}|g'
         statsd._send(stat, value_str, rate)
 
     def __getattr__(self, name: str) -> Any:
@@ -76,7 +76,7 @@ def run_in_batches(all_list: Sequence[T],
         batch = all_list[start:end]
 
         if logger:
-            logger("Executing %s in batch %s of %s" % (end-start, i+1, limit))
+            logger(f"Executing {end-start} in batch {i+1} of {limit}")
 
         callback(batch)
 
@@ -104,7 +104,7 @@ def log_statsd_event(name: str) -> None:
     Note that to draw this event as a vertical line in graphite
     you can use the drawAsInfinite() command
     """
-    event_name = "events.%s" % (name,)
+    event_name = f"events.{name}"
     statsd.incr(event_name)
 
 def generate_random_token(length: int) -> str:
diff --git a/zerver/lib/validator.py b/zerver/lib/validator.py
index 15dc93b075..00920b0706 100644
--- a/zerver/lib/validator.py
+++ b/zerver/lib/validator.py
@@ -221,7 +221,7 @@ def check_dict(required_keys: Iterable[Tuple[str, Validator]]=[],
             if k not in val:
                 return (_('%(key_name)s key is missing from %(var_name)s') %
                         {'key_name': k, 'var_name': var_name})
-            vname = '%s["%s"]' % (var_name, k)
+            vname = f'{var_name}["{k}"]'
             error = sub_validator(vname, val[k])
             if error:
                 return error
@@ -230,7 +230,7 @@ def check_dict(required_keys: Iterable[Tuple[str, Validator]]=[],
 
         for k, sub_validator in optional_keys:
             if k in val:
-                vname = '%s["%s"]' % (var_name, k)
+                vname = f'{var_name}["{k}"]'
                 error = sub_validator(vname, val[k])
                 if error:
                     return error
@@ -239,7 +239,7 @@ def check_dict(required_keys: Iterable[Tuple[str, Validator]]=[],
 
         if value_validator:
             for key in val:
-                vname = '%s contains a value that' % (var_name,)
+                vname = f'{var_name} contains a value that'
                 error = value_validator(vname, val[key])
                 if error:
                     return error
@@ -271,7 +271,7 @@ def check_variable_type(allowed_type_funcs: Iterable[Validator]) -> Validator:
     """
 
     if settings.LOG_API_EVENT_TYPES:
-        type_structure = 'any("%s")' % ([x.type_structure for x in allowed_type_funcs],)  # type: ignore[attr-defined] # monkey-patching
+        type_structure = f'any("{[x.type_structure for x in allowed_type_funcs]}")'  # type: ignore[attr-defined] # monkey-patching
     else:
         type_structure = None  # type: ignore[assignment] # monkey-patching
 
@@ -284,7 +284,7 @@ def check_variable_type(allowed_type_funcs: Iterable[Validator]) -> Validator:
     return enumerated_type_check
 
 def equals(expected_val: object) -> Validator:
-    @set_type_structure('equals("%s")' % (str(expected_val),))
+    @set_type_structure(f'equals("{str(expected_val)}")')
     def f(var_name: str, val: object) -> Optional[str]:
         if val != expected_val:
             return (_('%(variable)s != %(expected_value)s (%(value)s is wrong)') %
@@ -415,7 +415,7 @@ def to_non_negative_int(s: str, max_int_size: int=2**32-1) -> int:
     if x < 0:
         raise ValueError("argument is negative")
     if x > max_int_size:
-        raise ValueError('%s is too large (max %s)' % (x, max_int_size))
+        raise ValueError(f'{x} is too large (max {max_int_size})')
     return x
 
 def to_positive_or_allowed_int(allowed_integer: int) -> Callable[[str], int]:
diff --git a/zerver/lib/zephyr.py b/zerver/lib/zephyr.py
index 2d11536d9a..c39b2f52a7 100644
--- a/zerver/lib/zephyr.py
+++ b/zerver/lib/zephyr.py
@@ -8,7 +8,7 @@ def compute_mit_user_fullname(email: str) -> str:
         match_user = re.match(r'^([a-zA-Z0-9_.-]+)(\|.+)?@mit\.edu$', email.lower())
         if match_user and match_user.group(2) is None:
             answer = DNS.dnslookup(
-                "%s.passwd.ns.athena.mit.edu" % (match_user.group(1),),
+                f"{match_user.group(1)}.passwd.ns.athena.mit.edu",
                 DNS.Type.TXT)
             hesiod_name = answer[0][0].split(':')[4].split(',')[0].strip()
             if hesiod_name != "":
@@ -18,6 +18,6 @@ def compute_mit_user_fullname(email: str) -> str:
     except DNS.Base.ServerError:
         pass
     except Exception:
-        print("Error getting fullname for %s:" % (email,))
+        print(f"Error getting fullname for {email}:")
         traceback.print_exc()
     return email.lower()
diff --git a/zerver/management/commands/add_users_to_mailing_list.py b/zerver/management/commands/add_users_to_mailing_list.py
index 77826ca27a..8ea4b84ca6 100644
--- a/zerver/management/commands/add_users_to_mailing_list.py
+++ b/zerver/management/commands/add_users_to_mailing_list.py
@@ -66,6 +66,6 @@ class Command(BaseCommand):
             }
             r = requests.post(endpoint, auth=('apikey', api_key), json=data, timeout=10)
             if r.status_code == 400 and ujson.loads(r.text)['title'] == 'Member Exists':
-                print("%s is already a part of the list." % (data['email_address'],))
+                print("{} is already a part of the list.".format(data['email_address']))
             elif r.status_code >= 400:
                 print(r.text)
diff --git a/zerver/management/commands/add_users_to_streams.py b/zerver/management/commands/add_users_to_streams.py
index 7ed396aa86..04ecf73303 100644
--- a/zerver/management/commands/add_users_to_streams.py
+++ b/zerver/management/commands/add_users_to_streams.py
@@ -32,6 +32,6 @@ class Command(ZulipBaseCommand):
                 stream = ensure_stream(realm, stream_name)
                 _ignore, already_subscribed = bulk_add_subscriptions([stream], [user_profile])
                 was_there_already = user_profile.id in {tup[0].id for tup in already_subscribed}
-                print("%s %s to %s" % (
+                print("{} {} to {}".format(
                     "Already subscribed" if was_there_already else "Subscribed",
                     user_profile.delivery_email, stream_name))
diff --git a/zerver/management/commands/backup.py b/zerver/management/commands/backup.py
index f65776279d..a826cf62da 100644
--- a/zerver/management/commands/backup.py
+++ b/zerver/management/commands/backup.py
@@ -31,7 +31,7 @@ class Command(ZulipBaseCommand):
     def handle(self, *args: Any, **options: Any) -> None:
         timestamp = timezone_now().strftime(TIMESTAMP_FORMAT)
         with tempfile.TemporaryDirectory(
-            prefix="zulip-backup-%s-" % (timestamp,)
+            prefix=f"zulip-backup-{timestamp}-"
         ) as tmp:
             os.mkdir(os.path.join(tmp, "zulip-backup"))
             members = []
@@ -109,7 +109,7 @@ class Command(ZulipBaseCommand):
             try:
                 if options["output"] is None:
                     tarball_path = tempfile.NamedTemporaryFile(
-                        prefix="zulip-backup-%s-" % (timestamp,),
+                        prefix=f"zulip-backup-{timestamp}-",
                         suffix=".tar.gz",
                         delete=False,
                     ).name
@@ -122,7 +122,7 @@ class Command(ZulipBaseCommand):
                     + ["--"]
                     + members
                 )
-                print("Backup tarball written to %s" % (tarball_path,))
+                print(f"Backup tarball written to {tarball_path}")
             except BaseException:
                 if options["output"] is None:
                     os.unlink(tarball_path)
diff --git a/zerver/management/commands/bulk_change_user_name.py b/zerver/management/commands/bulk_change_user_name.py
index 970aceef9e..fe7abe71da 100644
--- a/zerver/management/commands/bulk_change_user_name.py
+++ b/zerver/management/commands/bulk_change_user_name.py
@@ -25,7 +25,7 @@ class Command(ZulipBaseCommand):
                 try:
                     user_profile = self.get_user(email, realm)
                     old_name = user_profile.full_name
-                    print("%s: %s -> %s" % (email, old_name, new_name))
+                    print(f"{email}: {old_name} -> {new_name}")
                     do_change_full_name(user_profile, new_name, None)
                 except CommandError:
-                    print("e-mail %s doesn't exist in the realm %s, skipping" % (email, realm))
+                    print(f"e-mail {email} doesn't exist in the realm {realm}, skipping")
diff --git a/zerver/management/commands/compilemessages.py b/zerver/management/commands/compilemessages.py
index e64dfb2e08..fff408548f 100644
--- a/zerver/management/commands/compilemessages.py
+++ b/zerver/management/commands/compilemessages.py
@@ -70,7 +70,7 @@ class Command(compilemessages.Command):
                     print(f"Problem in parsing {po_filename}")
                     raise
             else:
-                raise Exception("Unknown language %s" % (locale,))
+                raise Exception(f"Unknown language {locale}")
 
     def get_locales(self) -> List[str]:
         output = check_output(['git', 'ls-files', 'locale'])
diff --git a/zerver/management/commands/convert_gitter_data.py b/zerver/management/commands/convert_gitter_data.py
index a4fb54d8d2..62e88568c5 100644
--- a/zerver/management/commands/convert_gitter_data.py
+++ b/zerver/management/commands/convert_gitter_data.py
@@ -42,7 +42,7 @@ class Command(BaseCommand):
 
         for path in options['gitter_data']:
             if not os.path.exists(path):
-                raise CommandError("Gitter data file not found: '%s'" % (path,))
+                raise CommandError(f"Gitter data file not found: '{path}'")
             # TODO add json check
             print("Converting Data ...")
             do_convert_data(path, output_dir, num_threads)
diff --git a/zerver/management/commands/convert_hipchat_data.py b/zerver/management/commands/convert_hipchat_data.py
index 8182666337..9061c44766 100644
--- a/zerver/management/commands/convert_hipchat_data.py
+++ b/zerver/management/commands/convert_hipchat_data.py
@@ -72,7 +72,7 @@ class Command(BaseCommand):
 
         for path in options['hipchat_tar']:
             if not os.path.exists(path):
-                raise CommandError("Tar file not found: '%s'" % (path,))
+                raise CommandError(f"Tar file not found: '{path}'")
 
             print("Converting Data ...")
             do_convert_data(
diff --git a/zerver/management/commands/convert_mattermost_data.py b/zerver/management/commands/convert_mattermost_data.py
index 43ea478113..d0e1df3bdd 100644
--- a/zerver/management/commands/convert_mattermost_data.py
+++ b/zerver/management/commands/convert_mattermost_data.py
@@ -55,7 +55,7 @@ class Command(BaseCommand):
 
         data_dir = options['mattermost_data_dir']
         if not os.path.exists(data_dir):
-            raise CommandError("Directory not found: '%s'" % (data_dir,))
+            raise CommandError(f"Directory not found: '{data_dir}'")
         data_dir = os.path.realpath(data_dir)
 
         print("Converting Data ...")
diff --git a/zerver/management/commands/convert_slack_data.py b/zerver/management/commands/convert_slack_data.py
index 1cc9e21d83..7a75f0bbed 100644
--- a/zerver/management/commands/convert_slack_data.py
+++ b/zerver/management/commands/convert_slack_data.py
@@ -49,7 +49,7 @@ class Command(BaseCommand):
 
         for path in options['slack_data_zip']:
             if not os.path.exists(path):
-                raise CommandError("Slack data directory not found: '%s'" % (path,))
+                raise CommandError(f"Slack data directory not found: '{path}'")
 
             print("Converting Data ...")
             do_convert_data(path, output_dir, token, threads=num_threads)
diff --git a/zerver/management/commands/deactivate_user.py b/zerver/management/commands/deactivate_user.py
index d981a696d7..0914d45ef0 100644
--- a/zerver/management/commands/deactivate_user.py
+++ b/zerver/management/commands/deactivate_user.py
@@ -24,14 +24,12 @@ class Command(ZulipBaseCommand):
         realm = self.get_realm(options)
         user_profile = self.get_user(options['email'], realm)
 
-        print("Deactivating %s (%s) - %s" % (user_profile.full_name,
-                                             user_profile.delivery_email,
-                                             user_profile.realm.string_id))
-        print("%s has the following active sessions:" % (user_profile.delivery_email,))
+        print(f"Deactivating {user_profile.full_name} ({user_profile.delivery_email}) - {user_profile.realm.string_id}")
+        print(f"{user_profile.delivery_email} has the following active sessions:")
         for session in user_sessions(user_profile):
             print(session.expire_date, session.get_decoded())
         print("")
-        print("%s has %s active bots that will also be deactivated." % (
+        print("{} has {} active bots that will also be deactivated.".format(
             user_profile.delivery_email,
             UserProfile.objects.filter(
                 is_bot=True, is_active=True, bot_owner=user_profile
diff --git a/zerver/management/commands/delete_old_unclaimed_attachments.py b/zerver/management/commands/delete_old_unclaimed_attachments.py
index 8820e9cb6a..512f602114 100644
--- a/zerver/management/commands/delete_old_unclaimed_attachments.py
+++ b/zerver/management/commands/delete_old_unclaimed_attachments.py
@@ -27,12 +27,12 @@ class Command(BaseCommand):
 
     def handle(self, *args: Any, **options: Any) -> None:
         delta_weeks = options['delta_weeks']
-        print("Deleting unclaimed attached files older than %s weeks" % (delta_weeks,))
+        print(f"Deleting unclaimed attached files older than {delta_weeks} weeks")
 
         # print the list of files that are going to be removed
         old_attachments = get_old_unclaimed_attachments(delta_weeks)
         for old_attachment in old_attachments:
-            print("* %s created at %s" % (old_attachment.file_name, old_attachment.create_time))
+            print(f"* {old_attachment.file_name} created at {old_attachment.create_time}")
 
         print("")
         if not options["for_real"]:
diff --git a/zerver/management/commands/enqueue_file.py b/zerver/management/commands/enqueue_file.py
index 24bd1913b5..68ab4fda85 100644
--- a/zerver/management/commands/enqueue_file.py
+++ b/zerver/management/commands/enqueue_file.py
@@ -48,7 +48,7 @@ You can use "-" to represent stdin.
             except IndexError:
                 payload = line
 
-            print('Queueing to queue %s: %s' % (queue_name, payload))
+            print(f'Queueing to queue {queue_name}: {payload}')
 
             # Verify that payload is valid json.
             data = ujson.loads(payload)
diff --git a/zerver/management/commands/export.py b/zerver/management/commands/export.py
index 7a0a4c0c95..28752bf8f8 100644
--- a/zerver/management/commands/export.py
+++ b/zerver/management/commands/export.py
@@ -131,9 +131,9 @@ class Command(ZulipBaseCommand):
         try:
             os.close(os.open(tarball_path, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0o666))
         except FileExistsError:
-            raise CommandError("Refusing to overwrite existing tarball: %s. Aborting..." % (tarball_path,))
+            raise CommandError(f"Refusing to overwrite existing tarball: {tarball_path}. Aborting...")
 
-        print("\033[94mExporting realm\033[0m: %s" % (realm.string_id,))
+        print(f"\033[94mExporting realm\033[0m: {realm.string_id}")
 
         num_threads = int(options['threads'])
         if num_threads < 1:
diff --git a/zerver/management/commands/export_single_user.py b/zerver/management/commands/export_single_user.py
index 6f19a22a41..fbef9983f7 100644
--- a/zerver/management/commands/export_single_user.py
+++ b/zerver/management/commands/export_single_user.py
@@ -38,9 +38,9 @@ class Command(ZulipBaseCommand):
         if os.path.exists(output_dir):
             shutil.rmtree(output_dir)
         os.makedirs(output_dir)
-        print("Exporting user %s" % (user_profile.delivery_email,))
+        print(f"Exporting user {user_profile.delivery_email}")
         do_export_user(user_profile, output_dir)
-        print("Finished exporting to %s; tarring" % (output_dir,))
+        print(f"Finished exporting to {output_dir}; tarring")
         tarball_path = output_dir.rstrip('/') + '.tar.gz'
         subprocess.check_call(["tar", "--strip-components=1", "-czf", tarball_path, output_dir])
-        print("Tarball written to %s" % (tarball_path,))
+        print(f"Tarball written to {tarball_path}")
diff --git a/zerver/management/commands/fix_unreads.py b/zerver/management/commands/fix_unreads.py
index 0ad250ab64..22a841237a 100644
--- a/zerver/management/commands/fix_unreads.py
+++ b/zerver/management/commands/fix_unreads.py
@@ -41,7 +41,7 @@ class Command(ZulipBaseCommand):
             try:
                 user_profile = self.get_user(email, realm)
             except CommandError:
-                print("e-mail %s doesn't exist in the realm %s, skipping" % (email, realm))
+                print(f"e-mail {email} doesn't exist in the realm {realm}, skipping")
                 return
 
             fix(user_profile)
diff --git a/zerver/management/commands/generate_multiuse_invite_link.py b/zerver/management/commands/generate_multiuse_invite_link.py
index 3107aae443..6ee0839471 100644
--- a/zerver/management/commands/generate_multiuse_invite_link.py
+++ b/zerver/management/commands/generate_multiuse_invite_link.py
@@ -40,4 +40,4 @@ class Command(ZulipBaseCommand):
         referred_by = self.get_user(options['referred_by'], realm)
         invite_as = PreregistrationUser.INVITE_AS['MEMBER']
         invite_link = do_create_multiuse_invite_link(referred_by, invite_as, streams)
-        print("You can use %s to invite as many number of people to the organization." % (invite_link,))
+        print(f"You can use {invite_link} to invite as many number of people to the organization.")
diff --git a/zerver/management/commands/generate_realm_creation_link.py b/zerver/management/commands/generate_realm_creation_link.py
index 5bbc2ff0f2..d9fa493356 100644
--- a/zerver/management/commands/generate_realm_creation_link.py
+++ b/zerver/management/commands/generate_realm_creation_link.py
@@ -29,5 +29,5 @@ class Command(ZulipBaseCommand):
                                              "secure single-use link to register your "))
         self.stdout.write(self.style.SUCCESS("new Zulip organization:\033[0m"))
         self.stdout.write("")
-        self.stdout.write(self.style.SUCCESS("    \033[1;92m%s\033[0m" % (url,)))
+        self.stdout.write(self.style.SUCCESS(f"    \033[1;92m{url}\033[0m"))
         self.stdout.write("")
diff --git a/zerver/management/commands/import.py b/zerver/management/commands/import.py
index 5e77852e9d..1f3406ecd8 100644
--- a/zerver/management/commands/import.py
+++ b/zerver/management/commands/import.py
@@ -68,14 +68,14 @@ import a database dump from one or more JSON files."""
         for path in options['export_paths']:
             path = os.path.realpath(os.path.expanduser(path))
             if not os.path.exists(path):
-                raise CommandError("Directory not found: '%s'" % (path,))
+                raise CommandError(f"Directory not found: '{path}'")
             if not os.path.isdir(path):
                 raise CommandError("Export file should be folder; if it's a "
                                    "tarball, please unpack it first.")
             paths.append(path)
 
         for path in paths:
-            print("Processing dump: %s ..." % (path,))
+            print(f"Processing dump: {path} ...")
             realm = do_import_realm(path, subdomain, num_processes)
             print("Checking the system bots.")
             do_import_system_bots(realm)
diff --git a/zerver/management/commands/knight.py b/zerver/management/commands/knight.py
index 418b41bca9..601eb1908c 100644
--- a/zerver/management/commands/knight.py
+++ b/zerver/management/commands/knight.py
@@ -53,7 +53,7 @@ ONLY perform this on customer request from an authorized person.
                         do_change_user_role(user, UserProfile.ROLE_REALM_ADMINISTRATOR)
                     print("Done!")
                 else:
-                    print("Would have granted %s %s rights for %s" % (
+                    print("Would have granted {} {} rights for {}".format(
                           email, options['permission'], user.realm.string_id))
         else:
             if (user.is_realm_admin and options['permission'] == "administer" or
@@ -65,7 +65,7 @@ ONLY perform this on customer request from an authorized person.
                         do_change_user_role(user, UserProfile.ROLE_MEMBER)
                     print("Done!")
                 else:
-                    print("Would have removed %s's %s rights on %s" % (email, options['permission'],
-                                                                       user.realm.string_id))
+                    print("Would have removed {}'s {} rights on {}".format(email, options['permission'],
+                                                                           user.realm.string_id))
             else:
                 raise CommandError("User did not have permission for this realm!")
diff --git a/zerver/management/commands/makemessages.py b/zerver/management/commands/makemessages.py
index 3fa4040ba5..e3eec2a16e 100644
--- a/zerver/management/commands/makemessages.py
+++ b/zerver/management/commands/makemessages.py
@@ -40,10 +40,8 @@ from django.core.management.commands import makemessages
 from django.template.base import BLOCK_TAG_END, BLOCK_TAG_START
 from django.utils.translation import template
 
-strip_whitespace_right = re.compile("(%s-?\\s*(trans|pluralize).*?-%s)\\s+" % (
-                                    BLOCK_TAG_START, BLOCK_TAG_END), re.U)
-strip_whitespace_left = re.compile("\\s+(%s-\\s*(endtrans|pluralize).*?-?%s)" % (
-                                   BLOCK_TAG_START, BLOCK_TAG_END), re.U)
+strip_whitespace_right = re.compile(f"({BLOCK_TAG_START}-?\\s*(trans|pluralize).*?-{BLOCK_TAG_END})\\s+", re.U)
+strip_whitespace_left = re.compile(f"\\s+({BLOCK_TAG_START}-\\s*(endtrans|pluralize).*?-?{BLOCK_TAG_END})", re.U)
 
 regexes = [r'{{#tr .*?}}([\s\S]*?){{/tr}}',  # '.' doesn't match '\n' by default
            r'{{\s*t "(.*?)"\W*}}',
@@ -193,7 +191,7 @@ class Command(makemessages.Command):
         exclude = self.frontend_exclude
         process_all = self.frontend_all
 
-        paths = glob.glob('%s/*' % (self.default_locale_path,),)
+        paths = glob.glob(f'{self.default_locale_path}/*',)
         all_locales = [os.path.basename(path) for path in paths if os.path.isdir(path)]
 
         # Account for excluded locales
diff --git a/zerver/management/commands/merge_streams.py b/zerver/management/commands/merge_streams.py
index a5d9316c14..517446da72 100644
--- a/zerver/management/commands/merge_streams.py
+++ b/zerver/management/commands/merge_streams.py
@@ -46,7 +46,7 @@ class Command(ZulipBaseCommand):
         message_ids_to_clear = list(Message.objects.filter(
             recipient=recipient_to_destroy).values_list("id", flat=True))
         count = Message.objects.filter(recipient=recipient_to_destroy).update(recipient=recipient_to_keep)
-        print("Moved %s messages" % (count,))
+        print(f"Moved {count} messages")
         bulk_delete_cache_keys(message_ids_to_clear)
 
         # Move the Subscription objects.  This algorithm doesn't
@@ -62,11 +62,11 @@ class Command(ZulipBaseCommand):
         ]
 
         if len(subs_to_deactivate) > 0:
-            print("Deactivating %s subscriptions" % (len(subs_to_deactivate),))
+            print(f"Deactivating {len(subs_to_deactivate)} subscriptions")
             bulk_remove_subscriptions([sub.user_profile for sub in subs_to_deactivate],
                                       [stream_to_destroy],
                                       self.get_client())
         do_deactivate_stream(stream_to_destroy)
         if len(users_to_activate) > 0:
-            print("Adding %s subscriptions" % (len(users_to_activate),))
+            print(f"Adding {len(users_to_activate)} subscriptions")
             bulk_add_subscriptions([stream_to_keep], users_to_activate)
diff --git a/zerver/management/commands/purge_queue.py b/zerver/management/commands/purge_queue.py
index f8c44ef7c9..3cbe54e876 100644
--- a/zerver/management/commands/purge_queue.py
+++ b/zerver/management/commands/purge_queue.py
@@ -33,9 +33,9 @@ class Command(BaseCommand):
             queue_name = options['queue_name']
             if not (queue_name in get_active_worker_queues() or
                     queue_name.startswith("notify_tornado")):
-                raise CommandError("Unknown queue %s" % (queue_name,))
+                raise CommandError(f"Unknown queue {queue_name}")
 
-            print("Purging queue %s" % (queue_name,))
+            print(f"Purging queue {queue_name}")
             purge_queue(queue_name)
 
         print("Done")
diff --git a/zerver/management/commands/rate_limit.py b/zerver/management/commands/rate_limit.py
index 162c737e9e..4491351605 100644
--- a/zerver/management/commands/rate_limit.py
+++ b/zerver/management/commands/rate_limit.py
@@ -46,7 +46,7 @@ class Command(ZulipBaseCommand):
             try:
                 user_profile = get_user_profile_by_api_key(options['api_key'])
             except UserProfile.DoesNotExist:
-                raise CommandError("Unable to get user profile for api key %s" % (options['api_key'],))
+                raise CommandError("Unable to get user profile for api key {}".format(options['api_key']))
 
         users = [user_profile]
         if options['bots']:
@@ -55,7 +55,7 @@ class Command(ZulipBaseCommand):
 
         operation = options['operation']
         for user in users:
-            print("Applying operation to User ID: %s: %s" % (user.id, operation))
+            print(f"Applying operation to User ID: {user.id}: {operation}")
 
             if operation == 'block':
                 RateLimitedUser(user, domain=options['domain']).block_access(options['seconds'])
diff --git a/zerver/management/commands/realm_domain.py b/zerver/management/commands/realm_domain.py
index 77a5090f74..35b4c41e17 100644
--- a/zerver/management/commands/realm_domain.py
+++ b/zerver/management/commands/realm_domain.py
@@ -32,7 +32,7 @@ class Command(ZulipBaseCommand):
         realm = self.get_realm(options)
         assert realm is not None  # Should be ensured by parser
         if options["op"] == "show":
-            print("Domains for %s:" % (realm.string_id,))
+            print(f"Domains for {realm.string_id}:")
             for realm_domain in get_realm_domains(realm):
                 if realm_domain["allow_subdomains"]:
                     print(realm_domain["domain"] + " (subdomains allowed)")
diff --git a/zerver/management/commands/realm_filters.py b/zerver/management/commands/realm_filters.py
index c34fe55658..e4d4304685 100644
--- a/zerver/management/commands/realm_filters.py
+++ b/zerver/management/commands/realm_filters.py
@@ -38,7 +38,7 @@ Example: ./manage.py realm_filters --realm=zulip --op=show
         realm = self.get_realm(options)
         assert realm is not None  # Should be ensured by parser
         if options["op"] == "show":
-            print("%s: %s" % (realm.string_id, all_realm_filters().get(realm.id, [])))
+            print(f"{realm.string_id}: {all_realm_filters().get(realm.id, [])}")
             sys.exit(0)
 
         pattern = options['pattern']
diff --git a/zerver/management/commands/register_server.py b/zerver/management/commands/register_server.py
index 9fd68e4e43..31c39427a6 100644
--- a/zerver/management/commands/register_server.py
+++ b/zerver/management/commands/register_server.py
@@ -56,7 +56,7 @@ class Command(ZulipBaseCommand):
 
         print("The following data will be submitted to the push notification service:")
         for key in sorted(request.keys()):
-            print("  %s: %s" % (key, request[key]))
+            print(f"  {key}: {request[key]}")
         print("")
 
         if not options['agree_to_terms_of_service'] and not options["rotate_key"]:
@@ -88,7 +88,7 @@ class Command(ZulipBaseCommand):
             print("- Return to the documentation to learn how to test push notifications")
         else:
             if options["rotate_key"]:
-                print("Success! Updating %s with the new key..." % (SECRETS_FILENAME,))
+                print(f"Success! Updating {SECRETS_FILENAME} with the new key...")
                 subprocess.check_call(["crudini", '--set', SECRETS_FILENAME, "secrets", "zulip_org_key",
                                        request["new_org_key"]])
             print("Mobile Push Notification Service registration successfully updated!")
diff --git a/zerver/management/commands/remove_users_from_stream.py b/zerver/management/commands/remove_users_from_stream.py
index 743eb97e34..a646b250c3 100644
--- a/zerver/management/commands/remove_users_from_stream.py
+++ b/zerver/management/commands/remove_users_from_stream.py
@@ -33,6 +33,6 @@ class Command(ZulipBaseCommand):
 
         for user_profile in user_profiles:
             if user_profile in not_subscribed_users:
-                print("%s was not subscribed" % (user_profile.delivery_email,))
+                print(f"{user_profile.delivery_email} was not subscribed")
             else:
-                print("Removed %s from %s" % (user_profile.delivery_email, stream_name))
+                print(f"Removed {user_profile.delivery_email} from {stream_name}")
diff --git a/zerver/management/commands/runtornado.py b/zerver/management/commands/runtornado.py
index beb09355fc..02e27760c0 100644
--- a/zerver/management/commands/runtornado.py
+++ b/zerver/management/commands/runtornado.py
@@ -65,7 +65,7 @@ class Command(BaseCommand):
             addr = '127.0.0.1'
 
         if not port.isdigit():
-            raise CommandError("%r is not a valid port number." % (port,))
+            raise CommandError(f"{port!r} is not a valid port number.")
 
         xheaders = options.get('xheaders', True)
         no_keep_alive = options.get('no_keep_alive', False)
@@ -82,7 +82,7 @@ class Command(BaseCommand):
             # We pass display_num_errors=False, since Django will
             # likely display similar output anyway.
             self.check(display_num_errors=False)
-            print("Tornado server is running at http://%s:%s/" % (addr, port))
+            print(f"Tornado server is running at http://{addr}:{port}/")
 
             if settings.USING_RABBITMQ:
                 queue_client = get_queue_client()
diff --git a/zerver/management/commands/send_realm_reactivation_email.py b/zerver/management/commands/send_realm_reactivation_email.py
index 5ff1f60015..25c2e67cb2 100644
--- a/zerver/management/commands/send_realm_reactivation_email.py
+++ b/zerver/management/commands/send_realm_reactivation_email.py
@@ -15,7 +15,7 @@ class Command(ZulipBaseCommand):
         realm = self.get_realm(options)
         assert realm is not None
         if not realm.deactivated:
-            raise CommandError("The realm %s is already active." % (realm.name,))
+            raise CommandError(f"The realm {realm.name} is already active.")
         print('Sending email to admins')
         do_send_realm_reactivation_email(realm)
         print('Done!')
diff --git a/zerver/management/commands/send_test_email.py b/zerver/management/commands/send_test_email.py
index ce512afcdb..58016d5f2a 100644
--- a/zerver/management/commands/send_test_email.py
+++ b/zerver/management/commands/send_test_email.py
@@ -32,13 +32,13 @@ class Command(sendtestemail.Command):
                    "the Zulip server with /home/zulip/deployments/current/scripts/restart-server "
                    "after changing the settings in /etc/zulip before your changes will take effect.")
         sender = FromAddress.SUPPORT
-        print("  * %s" % (sender,))
+        print(f"  * {sender}")
         send_mail("Zulip email test", message, sender, kwargs['email'])
         noreply_sender = FromAddress.tokenized_no_reply_address()
-        print("  * %s" % (noreply_sender,))
+        print(f"  * {noreply_sender}")
         send_mail("Zulip noreply email test", message, noreply_sender, kwargs['email'])
         print()
-        print("Successfully sent 2 emails to %s!" % (", ".join(kwargs['email']),))
+        print("Successfully sent 2 emails to {}!".format(", ".join(kwargs['email'])))
 
         if kwargs['managers']:
             mail_managers("Zulip manager email test", "This email was sent to the site managers.")
diff --git a/zerver/management/commands/send_webhook_fixture_message.py b/zerver/management/commands/send_webhook_fixture_message.py
index b12b8761be..9da940be72 100644
--- a/zerver/management/commands/send_webhook_fixture_message.py
+++ b/zerver/management/commands/send_webhook_fixture_message.py
@@ -85,7 +85,7 @@ approach shown above.
             result = client.post(options['url'], json, content_type="application/json",
                                  HTTP_HOST=realm.host)
         if result.status_code != 200:
-            raise CommandError('Error status %s: %s' % (result.status_code, result.content))
+            raise CommandError(f'Error status {result.status_code}: {result.content}')
 
     def _does_fixture_path_exist(self, fixture_path: str) -> bool:
         return os.path.exists(fixture_path)
diff --git a/zerver/management/commands/turn_off_digests.py b/zerver/management/commands/turn_off_digests.py
index a7d7716c38..ee68e18f92 100644
--- a/zerver/management/commands/turn_off_digests.py
+++ b/zerver/management/commands/turn_off_digests.py
@@ -26,5 +26,4 @@ class Command(ZulipBaseCommand):
                 do_change_notification_settings(user_profile, 'enable_digest_emails', False)
             else:
                 already_disabled_prefix = "(already off) "
-            print("%s%s <%s>" % (already_disabled_prefix, user_profile.full_name,
-                                 user_profile.delivery_email))
+            print(f"{already_disabled_prefix}{user_profile.full_name} <{user_profile.delivery_email}>")
diff --git a/zerver/middleware.py b/zerver/middleware.py
index dc9b914205..d19f35e3df 100644
--- a/zerver/middleware.py
+++ b/zerver/middleware.py
@@ -76,8 +76,8 @@ def timedelta_ms(timedelta: float) -> float:
 
 def format_timedelta(timedelta: float) -> str:
     if (timedelta >= 1):
-        return "%.1fs" % (timedelta,)
-    return "%.0fms" % (timedelta_ms(timedelta),)
+        return f"{timedelta:.1f}s"
+    return f"{timedelta_ms(timedelta):.0f}ms"
 
 def is_slow_query(time_delta: float, path: str) -> bool:
     if time_delta < 1.2:
@@ -114,7 +114,7 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
         if path == '/':
             statsd_path = 'webreq'
         else:
-            statsd_path = "webreq.%s" % (path[1:].replace('/', '.'),)
+            statsd_path = "webreq.{}".format(path[1:].replace('/', '.'))
             # Remove non-ascii chars from path (there should be none, if there are it's
             # because someone manually entered a nonexistent path), as UTF-8 chars make
             # statsd sad when it sends the key name over the socket
@@ -135,7 +135,7 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
         orig_time_delta = time_delta
         time_delta = ((log_data['time_stopped'] - log_data['time_started']) +
                       (time.time() - log_data['time_restarted']))
-        optional_orig_delta = " (lp: %s)" % (format_timedelta(orig_time_delta),)
+        optional_orig_delta = f" (lp: {format_timedelta(orig_time_delta)})"
     remote_cache_output = ""
     if 'remote_cache_time_start' in log_data:
         remote_cache_time_delta = get_remote_cache_time() - log_data['remote_cache_time_start']
@@ -148,16 +148,15 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
                                          log_data['remote_cache_requests_restarted'])
 
         if (remote_cache_time_delta > 0.005):
-            remote_cache_output = " (mem: %s/%s)" % (format_timedelta(remote_cache_time_delta),
-                                                     remote_cache_count_delta)
+            remote_cache_output = f" (mem: {format_timedelta(remote_cache_time_delta)}/{remote_cache_count_delta})"
 
         if not suppress_statsd:
-            statsd.timing("%s.remote_cache.time" % (statsd_path,), timedelta_ms(remote_cache_time_delta))
-            statsd.incr("%s.remote_cache.querycount" % (statsd_path,), remote_cache_count_delta)
+            statsd.timing(f"{statsd_path}.remote_cache.time", timedelta_ms(remote_cache_time_delta))
+            statsd.incr(f"{statsd_path}.remote_cache.querycount", remote_cache_count_delta)
 
     startup_output = ""
     if 'startup_time_delta' in log_data and log_data["startup_time_delta"] > 0.005:
-        startup_output = " (+start: %s)" % (format_timedelta(log_data["startup_time_delta"]),)
+        startup_output = " (+start: {})".format(format_timedelta(log_data["startup_time_delta"]))
 
     bugdown_output = ""
     if 'bugdown_time_start' in log_data:
@@ -171,32 +170,30 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
                                     log_data['bugdown_requests_restarted'])
 
         if (bugdown_time_delta > 0.005):
-            bugdown_output = " (md: %s/%s)" % (format_timedelta(bugdown_time_delta),
-                                               bugdown_count_delta)
+            bugdown_output = f" (md: {format_timedelta(bugdown_time_delta)}/{bugdown_count_delta})"
 
             if not suppress_statsd:
-                statsd.timing("%s.markdown.time" % (statsd_path,), timedelta_ms(bugdown_time_delta))
-                statsd.incr("%s.markdown.count" % (statsd_path,), bugdown_count_delta)
+                statsd.timing(f"{statsd_path}.markdown.time", timedelta_ms(bugdown_time_delta))
+                statsd.incr(f"{statsd_path}.markdown.count", bugdown_count_delta)
 
     # Get the amount of time spent doing database queries
     db_time_output = ""
     queries = connection.connection.queries if connection.connection is not None else []
     if len(queries) > 0:
         query_time = sum(float(query.get('time', 0)) for query in queries)
-        db_time_output = " (db: %s/%sq)" % (format_timedelta(query_time),
-                                            len(queries))
+        db_time_output = f" (db: {format_timedelta(query_time)}/{len(queries)}q)"
 
         if not suppress_statsd:
             # Log ms, db ms, and num queries to statsd
-            statsd.timing("%s.dbtime" % (statsd_path,), timedelta_ms(query_time))
-            statsd.incr("%s.dbq" % (statsd_path,), len(queries))
-            statsd.timing("%s.total" % (statsd_path,), timedelta_ms(time_delta))
+            statsd.timing(f"{statsd_path}.dbtime", timedelta_ms(query_time))
+            statsd.incr(f"{statsd_path}.dbq", len(queries))
+            statsd.timing(f"{statsd_path}.total", timedelta_ms(time_delta))
 
     if 'extra' in log_data:
-        extra_request_data = " %s" % (log_data['extra'],)
+        extra_request_data = " {}".format(log_data['extra'])
     else:
         extra_request_data = ""
-    logger_client = "(%s via %s)" % (requestor_for_logs, client_name)
+    logger_client = f"({requestor_for_logs} via {client_name})"
     logger_timing = ('%5s%s%s%s%s%s %s' %
                      (format_timedelta(time_delta), optional_orig_delta,
                       remote_cache_output, bugdown_output,
@@ -214,7 +211,7 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
 
     if settings.PROFILE_ALL_REQUESTS:
         log_data["prof"].disable()
-        profile_path = "/tmp/profile.data.%s.%s" % (path.split("/")[-1], int(time_delta * 1000),)
+        profile_path = "/tmp/profile.data.{}.{}".format(path.split("/")[-1], int(time_delta * 1000))
         log_data["prof"].dump_stats(profile_path)
 
     # Log some additional data whenever we return certain 40x errors
@@ -284,7 +281,7 @@ class LogRequests(MiddlewareMixin):
             if hasattr(request, 'user') and hasattr(request.user, 'format_requestor_for_logs'):
                 requestor_for_logs = request.user.format_requestor_for_logs()
             else:
-                requestor_for_logs = "unauth@%s" % (get_subdomain(request) or 'root',)
+                requestor_for_logs = "unauth@{}".format(get_subdomain(request) or 'root')
         try:
             client = request.client.name
         except Exception:
diff --git a/zerver/migrations/0037_disallow_null_string_id.py b/zerver/migrations/0037_disallow_null_string_id.py
index a4a8a86c70..823aefe925 100644
--- a/zerver/migrations/0037_disallow_null_string_id.py
+++ b/zerver/migrations/0037_disallow_null_string_id.py
@@ -22,7 +22,7 @@ def set_string_id_using_domain(apps: StateApps, schema_editor: DatabaseSchemaEdi
                     continue
                 except IntegrityError:
                     pass
-            raise RuntimeError("Unable to find a good string_id for realm %s" % (realm,))
+            raise RuntimeError(f"Unable to find a good string_id for realm {realm}")
 
 class Migration(migrations.Migration):
 
diff --git a/zerver/migrations/0177_user_message_add_and_index_is_private_flag.py b/zerver/migrations/0177_user_message_add_and_index_is_private_flag.py
index 637a23bf9d..efc0d92ea1 100644
--- a/zerver/migrations/0177_user_message_add_and_index_is_private_flag.py
+++ b/zerver/migrations/0177_user_message_add_and_index_is_private_flag.py
@@ -42,7 +42,7 @@ def reset_is_private_flag(
         i += 1
         if (i % 50 == 0 or i == total):
             percent = round((i / total) * 100, 2)
-            print("Processed %s/%s %s%%" % (i, total, percent))
+            print(f"Processed {i}/{total} {percent}%")
             sys.stdout.flush()
 
 class Migration(migrations.Migration):
diff --git a/zerver/migrations/0182_set_initial_value_is_private_flag.py b/zerver/migrations/0182_set_initial_value_is_private_flag.py
index c326109d58..4b40daa396 100644
--- a/zerver/migrations/0182_set_initial_value_is_private_flag.py
+++ b/zerver/migrations/0182_set_initial_value_is_private_flag.py
@@ -36,7 +36,7 @@ def set_initial_value_of_is_private_flag(
             percent = round((processed / total) * 100, 2)
         else:
             percent = 100.00
-        print("Processed %s/%s %s%%" % (processed, total, percent))
+        print(f"Processed {processed}/{total} {percent}%")
         sys.stdout.flush()
 
 class Migration(migrations.Migration):
diff --git a/zerver/migrations/0257_fix_has_link_attribute.py b/zerver/migrations/0257_fix_has_link_attribute.py
index 4370749541..8f031795f4 100644
--- a/zerver/migrations/0257_fix_has_link_attribute.py
+++ b/zerver/migrations/0257_fix_has_link_attribute.py
@@ -22,7 +22,7 @@ def process_batch(apps: StateApps, id_start: int, id_end: int, last_id: int) ->
             continue
 
         if message.id % 1000 == 0:
-            print("Processed %s / %s" % (message.id, last_id))
+            print(f"Processed {message.id} / {last_id}")
 
         # Because we maintain the Attachment table, this should be as
         # simple as just just checking if there's any Attachment
diff --git a/zerver/models.py b/zerver/models.py
index b86b0cf1bc..5d65b10baa 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -102,10 +102,10 @@ def get_display_recipient(recipient: 'Recipient') -> DisplayRecipientT:
     )
 
 def get_realm_emoji_cache_key(realm: 'Realm') -> str:
-    return 'realm_emoji:%s' % (realm.id,)
+    return f'realm_emoji:{realm.id}'
 
 def get_active_realm_emoji_cache_key(realm: 'Realm') -> str:
-    return 'active_realm_emoji:%s' % (realm.id,)
+    return f'active_realm_emoji:{realm.id}'
 
 # This simple call-once caching saves ~500us in auth_enabled_helper,
 # which is a significant optimization for common_context.  Note that
@@ -420,7 +420,7 @@ class Realm(models.Model):
         return ret
 
     def __str__(self) -> str:
-        return "<Realm: %s %s>" % (self.string_id, self.id)
+        return f"<Realm: {self.string_id} {self.id}>"
 
     @cache_with_key(get_realm_emoji_cache_key, timeout=3600*24*7)
     def get_emoji(self) -> Dict[str, Dict[str, Iterable[str]]]:
@@ -524,7 +524,7 @@ class Realm(models.Model):
     def host_for_subdomain(subdomain: str) -> str:
         if subdomain == Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
             return settings.EXTERNAL_HOST
-        default_host = "%s.%s" % (subdomain, settings.EXTERNAL_HOST)
+        default_host = f"{subdomain}.{settings.EXTERNAL_HOST}"
         return settings.REALM_HOSTS.get(subdomain, default_host)
 
     @property
@@ -614,11 +614,7 @@ class RealmEmoji(models.Model):
     PATH_ID_TEMPLATE = "{realm_id}/emoji/images/{emoji_file_name}"
 
     def __str__(self) -> str:
-        return "<RealmEmoji(%s): %s %s %s %s>" % (self.realm.string_id,
-                                                  self.id,
-                                                  self.name,
-                                                  self.deactivated,
-                                                  self.file_name)
+        return f"<RealmEmoji({self.realm.string_id}): {self.id} {self.name} {self.deactivated} {self.file_name}>"
 
 def get_realm_emoji_dicts(realm: Realm,
                           only_active_emojis: bool=False) -> Dict[str, Dict[str, Any]]:
@@ -694,10 +690,10 @@ class RealmFilter(models.Model):
         unique_together = ("realm", "pattern")
 
     def __str__(self) -> str:
-        return "<RealmFilter(%s): %s %s>" % (self.realm.string_id, self.pattern, self.url_format_string)
+        return f"<RealmFilter({self.realm.string_id}): {self.pattern} {self.url_format_string}>"
 
 def get_realm_filters_cache_key(realm_id: int) -> str:
-    return '%s:all_realm_filters:%s' % (cache.KEY_PREFIX, realm_id,)
+    return f'{cache.KEY_PREFIX}:all_realm_filters:{realm_id}'
 
 # We have a per-process cache to avoid doing 1000 remote cache queries during page load
 per_request_realm_filters_cache: Dict[int, List[Tuple[str, str, int]]] = {}
@@ -1104,7 +1100,7 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
             return False
 
     def __str__(self) -> str:
-        return "<UserProfile: %s %s>" % (self.email, self.realm)
+        return f"<UserProfile: {self.email} {self.realm}>"
 
     @property
     def is_new_member(self) -> bool:
@@ -1432,7 +1428,7 @@ class Stream(models.Model):
     first_message_id: Optional[int] = models.IntegerField(null=True, db_index=True)
 
     def __str__(self) -> str:
-        return "<Stream: %s>" % (self.name,)
+        return f"<Stream: {self.name}>"
 
     def is_public(self) -> bool:
         # All streams are private in Zephyr mirroring realms.
@@ -1502,16 +1498,13 @@ class MutedTopic(models.Model):
         unique_together = ('user_profile', 'stream', 'topic_name')
 
     def __str__(self) -> str:
-        return ("<MutedTopic: (%s, %s, %s, %s)>" % (self.user_profile.email,
-                                                    self.stream.name,
-                                                    self.topic_name,
-                                                    self.date_muted))
+        return (f"<MutedTopic: ({self.user_profile.email}, {self.stream.name}, {self.topic_name}, {self.date_muted})>")
 
 class Client(models.Model):
     name: str = models.CharField(max_length=30, db_index=True, unique=True)
 
     def __str__(self) -> str:
-        return "<Client: %s>" % (self.name,)
+        return f"<Client: {self.name}>"
 
 get_client_cache: Dict[str, Client] = {}
 def get_client(name: str) -> Client:
@@ -1524,7 +1517,7 @@ def get_client(name: str) -> Client:
     return get_client_cache[cache_name]
 
 def get_client_cache_key(name: str) -> str:
-    return 'get_client:%s' % (make_safe_digest(name),)
+    return f'get_client:{make_safe_digest(name)}'
 
 @cache_with_key(get_client_cache_key, timeout=3600*24*7)
 def get_client_remote_cache(name: str) -> Client:
@@ -1660,8 +1653,7 @@ class AbstractMessage(models.Model):
 
     def __str__(self) -> str:
         display_recipient = get_display_recipient(self.recipient)
-        return "<%s: %s / %s / %s>" % (self.__class__.__name__, display_recipient,
-                                       self.subject, self.sender)
+        return f"<{self.__class__.__name__}: {display_recipient} / {self.subject} / {self.sender}>"
 
 class ArchiveTransaction(models.Model):
     timestamp: datetime.datetime = models.DateTimeField(default=timezone_now, db_index=True)
@@ -1859,7 +1851,7 @@ class Reaction(AbstractReaction):
         return Reaction.objects.filter(message_id__in=needed_ids).values(*fields)
 
     def __str__(self) -> str:
-        return "%s / %s / %s" % (self.user_profile.email, self.message.id, self.emoji_name)
+        return f"{self.user_profile.email} / {self.message.id} / {self.emoji_name}"
 
 class ArchivedReaction(AbstractReaction):
     message: ArchivedMessage = models.ForeignKey(ArchivedMessage, on_delete=CASCADE)
@@ -1973,8 +1965,7 @@ class AbstractUserMessage(models.Model):
 
     def __str__(self) -> str:
         display_recipient = get_display_recipient(self.message.recipient)
-        return "<%s: %s / %s (%s)>" % (self.__class__.__name__, display_recipient,
-                                       self.user_profile.email, self.flags_list())
+        return f"<{self.__class__.__name__}: {display_recipient} / {self.user_profile.email} ({self.flags_list()})>"
 
 
 class UserMessage(AbstractUserMessage):
@@ -2020,7 +2011,7 @@ class AbstractAttachment(models.Model):
         abstract = True
 
     def __str__(self) -> str:
-        return "<%s: %s>" % (self.__class__.__name__, self.file_name,)
+        return f"<{self.__class__.__name__}: {self.file_name}>"
 
 
 class ArchivedAttachment(AbstractAttachment):
@@ -2128,7 +2119,7 @@ class Subscription(models.Model):
         unique_together = ("user_profile", "recipient")
 
     def __str__(self) -> str:
-        return "<Subscription: %s -> %s>" % (self.user_profile, self.recipient)
+        return f"<Subscription: {self.user_profile} -> {self.recipient}>"
 
     # Subscription fields included whenever a Subscription object is provided to
     # Zulip clients via the API.  A few details worth noting:
@@ -2329,7 +2320,7 @@ def get_huddle_hash(id_list: List[int]) -> str:
     return make_safe_digest(hash_key)
 
 def huddle_hash_cache_key(huddle_hash: str) -> str:
-    return "huddle_by_hash:%s" % (huddle_hash,)
+    return f"huddle_by_hash:{huddle_hash}"
 
 def get_huddle(id_list: List[int]) -> Huddle:
     huddle_hash = get_huddle_hash(id_list)
@@ -2413,7 +2404,7 @@ class UserPresence(models.Model):
         elif status == UserPresence.IDLE:
             return 'idle'
         else:  # nocoverage # TODO: Add a presence test to cover this.
-            raise ValueError('Unknown status: %s' % (status,))
+            raise ValueError(f'Unknown status: {status}')
 
     @staticmethod
     def to_presence_dict(client_name: str, status: int, dt: datetime.datetime, push_enabled: bool=False,
@@ -2510,9 +2501,7 @@ class ScheduledEmail(AbstractScheduledJob):
     type: int = models.PositiveSmallIntegerField()
 
     def __str__(self) -> str:
-        return "<ScheduledEmail: %s %s %s>" % (self.type,
-                                               self.address or list(self.users.all()),
-                                               self.scheduled_timestamp)
+        return f"<ScheduledEmail: {self.type} {self.address or list(self.users.all())} {self.scheduled_timestamp}>"
 
 class MissedMessageEmailAddress(models.Model):
     EXPIRY_SECONDS = 60 * 60 * 24 * 5
@@ -2570,9 +2559,7 @@ class ScheduledMessage(models.Model):
 
     def __str__(self) -> str:
         display_recipient = get_display_recipient(self.recipient)
-        return "<ScheduledMessage: %s %s %s %s>" % (display_recipient,
-                                                    self.subject, self.sender,
-                                                    self.scheduled_timestamp)
+        return f"<ScheduledMessage: {display_recipient} {self.subject} {self.sender} {self.scheduled_timestamp}>"
 
 EMAIL_TYPES = {
     'followup_day1': ScheduledEmail.WELCOME,
@@ -2680,13 +2667,10 @@ class RealmAuditLog(AbstractRealmAuditLog):
 
     def __str__(self) -> str:
         if self.modified_user is not None:
-            return "<RealmAuditLog: %s %s %s %s>" % (
-                self.modified_user, self.event_type, self.event_time, self.id)
+            return f"<RealmAuditLog: {self.modified_user} {self.event_type} {self.event_time} {self.id}>"
         if self.modified_stream is not None:
-            return "<RealmAuditLog: %s %s %s %s>" % (
-                self.modified_stream, self.event_type, self.event_time, self.id)
-        return "<RealmAuditLog: %s %s %s %s>" % (
-            self.realm, self.event_type, self.event_time, self.id)
+            return f"<RealmAuditLog: {self.modified_stream} {self.event_type} {self.event_time} {self.id}>"
+        return f"<RealmAuditLog: {self.realm} {self.event_type} {self.event_time} {self.id}>"
 
 class UserHotspot(models.Model):
     user: UserProfile = models.ForeignKey(UserProfile, on_delete=CASCADE)
@@ -2827,7 +2811,7 @@ class CustomProfileFieldValue(models.Model):
         unique_together = ('user_profile', 'field')
 
     def __str__(self) -> str:
-        return "<CustomProfileFieldValue: %s %s %s>" % (self.user_profile, self.field, self.value)
+        return f"<CustomProfileFieldValue: {self.user_profile} {self.field} {self.value}>"
 
 # Interfaces for services
 # They provide additional functionality like parsing message to obtain query url, data to be sent to url,
diff --git a/zerver/openapi/markdown_extension.py b/zerver/openapi/markdown_extension.py
index 19f4ecb3cd..4c692ee344 100644
--- a/zerver/openapi/markdown_extension.py
+++ b/zerver/openapi/markdown_extension.py
@@ -269,7 +269,7 @@ def generate_curl_example(endpoint: str, method: str,
         raise AssertionError("Unhandled securityScheme. Please update the code to handle this scheme.")
 
     if authentication_required:
-        lines.append("    -u %s:%s" % (auth_email, auth_api_key))
+        lines.append(f"    -u {auth_email}:{auth_api_key}")
 
     for param in operation_params:
         if param["in"] == "path":
diff --git a/zerver/openapi/python_examples.py b/zerver/openapi/python_examples.py
index b32bfa58c2..34bb899194 100644
--- a/zerver/openapi/python_examples.py
+++ b/zerver/openapi/python_examples.py
@@ -888,7 +888,7 @@ def upload_file(client: Client) -> None:
         "type": "stream",
         "to": "Denmark",
         "topic": "Castle",
-        "content": "Check out [this picture](%s) of my castle!" % (result['uri'],)
+        "content": "Check out [this picture]({}) of my castle!".format(result['uri'])
     })
     # {code_example|end}
 
diff --git a/zerver/openapi/test_curl_examples.py b/zerver/openapi/test_curl_examples.py
index 2fea31bbfb..9d1ec803ce 100644
--- a/zerver/openapi/test_curl_examples.py
+++ b/zerver/openapi/test_curl_examples.py
@@ -39,7 +39,7 @@ def test_generated_curl_examples_for_success(client: Client) -> None:
             curl_command_text = curl_command_text.replace(
                 "BOT_EMAIL_ADDRESS:BOT_API_KEY", authentication_line)
 
-            print("Testing %s ..." % (curl_command_text.split("\n")[0],))
+            print("Testing {} ...".format(curl_command_text.split("\n")[0]))
 
             # Turn the text into an arguments list.
             generated_curl_command = [
diff --git a/zerver/templatetags/app_filters.py b/zerver/templatetags/app_filters.py
index 9683a340e2..094c98c9ee 100644
--- a/zerver/templatetags/app_filters.py
+++ b/zerver/templatetags/app_filters.py
@@ -44,16 +44,16 @@ def display_list(values: List[str], display_limit: int) -> str:
     """
     if len(values) == 1:
         # One value, show it.
-        display_string = "%s" % (values[0],)
+        display_string = f"{values[0]}"
     elif len(values) <= display_limit:
         # Fewer than `display_limit` values, show all of them.
         display_string = ", ".join(
-            "%s" % (value,) for value in values[:-1])
-        display_string += " and %s" % (values[-1],)
+            f"{value}" for value in values[:-1])
+        display_string += f" and {values[-1]}"
     else:
         # More than `display_limit` values, only mention a few.
         display_string = ", ".join(
-            "%s" % (value,) for value in values[:display_limit])
+            f"{value}" for value in values[:display_limit])
         display_string += and_n_others(values, display_limit)
 
     return display_string
diff --git a/zerver/tests/test_audit_log.py b/zerver/tests/test_audit_log.py
index 6d11dac0f5..34ed744067 100644
--- a/zerver/tests/test_audit_log.py
+++ b/zerver/tests/test_audit_log.py
@@ -97,7 +97,7 @@ class TestRealmAuditLog(ZulipTestCase):
 
         # Test the RealmAuditLog stringification
         audit_entry = RealmAuditLog.objects.get(event_type=RealmAuditLog.USER_EMAIL_CHANGED, event_time__gte=now)
-        self.assertTrue(str(audit_entry).startswith("<RealmAuditLog: <UserProfile: %s %s> %s " % (user.email, user.realm, RealmAuditLog.USER_EMAIL_CHANGED)))
+        self.assertTrue(str(audit_entry).startswith(f"<RealmAuditLog: <UserProfile: {user.email} {user.realm}> {RealmAuditLog.USER_EMAIL_CHANGED} "))
 
     def test_change_avatar_source(self) -> None:
         now = timezone_now()
diff --git a/zerver/tests/test_auth_backends.py b/zerver/tests/test_auth_backends.py
index 239f61ee07..aeffd24eb7 100644
--- a/zerver/tests/test_auth_backends.py
+++ b/zerver/tests/test_auth_backends.py
@@ -621,7 +621,7 @@ class DesktopFlowTestingLib(ZulipTestCase):
         browser_url = soup.find("a", href=True)["href"]
 
         decrypted_key = self.verify_desktop_data_and_return_key(desktop_data, desktop_flow_otp)
-        self.assertEqual(browser_url, 'http://zulip.testserver/accounts/login/subdomain/%s' % (decrypted_key,))
+        self.assertEqual(browser_url, f'http://zulip.testserver/accounts/login/subdomain/{decrypted_key}')
 
         result = self.client_get(browser_url)
         self.assertEqual(result.status_code, 302)
@@ -706,7 +706,7 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase):
         params['next'] = next
         params['multiuse_object_key'] = multiuse_object_key
         if len(params) > 0:
-            url += "?%s" % (urllib.parse.urlencode(params),)
+            url += f"?{urllib.parse.urlencode(params)}"
         if user_agent is not None:
             headers['HTTP_USER_AGENT'] = user_agent
 
@@ -773,7 +773,7 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase):
 
         result = self.client_get(url, **headers)
 
-        expected_result_url_prefix = 'http://testserver/login/%s/' % (self.backend.name,)
+        expected_result_url_prefix = f'http://testserver/login/{self.backend.name}/'
         if settings.SOCIAL_AUTH_SUBDOMAIN is not None:
             expected_result_url_prefix = ('http://%s.testserver/login/%s/' %
                                           (settings.SOCIAL_AUTH_SUBDOMAIN, self.backend.name,))
@@ -1405,10 +1405,10 @@ class SAMLAuthBackendTest(SocialAuthBase):
 
         result = self.client_get(url, **headers)
 
-        expected_result_url_prefix = 'http://testserver/login/%s/' % (self.backend.name,)
+        expected_result_url_prefix = f'http://testserver/login/{self.backend.name}/'
         if settings.SOCIAL_AUTH_SUBDOMAIN is not None:
             expected_result_url_prefix = (
-                'http://%s.testserver/login/%s/' % (settings.SOCIAL_AUTH_SUBDOMAIN, self.backend.name)
+                f'http://{settings.SOCIAL_AUTH_SUBDOMAIN}.testserver/login/{self.backend.name}/'
             )
 
         if result.status_code != 302 or not result.url.startswith(expected_result_url_prefix):
@@ -1600,7 +1600,7 @@ class SAMLAuthBackendTest(SocialAuthBase):
             self.assertEqual(result.status_code, 302)
             self.assertEqual('/login/', result.url)
         self.assertEqual(m.output, [self.logger_output(
-            "/complete/saml/: Can't figure out subdomain for this authentication request. relayed_params: %s" % ("{}",),
+            "/complete/saml/: Can't figure out subdomain for this authentication request. relayed_params: {}".format("{}"),
             'info'
         )])
 
@@ -1648,13 +1648,13 @@ class SAMLAuthBackendTest(SocialAuthBase):
             result = self.client_get('/login/saml/')
             self.assertEqual(result.status_code, 302)
             self.assertEqual('/login/', result.url)
-        self.assertEqual(m.output, [self.logger_output("/login/saml/ : Bad idp param: KeyError: %s." % ("'idp'",), 'info')])
+        self.assertEqual(m.output, [self.logger_output("/login/saml/ : Bad idp param: KeyError: {}.".format("'idp'"), 'info')])
 
         with self.assertLogs(self.logger_string, level='INFO') as m:
             result = self.client_get('/login/saml/?idp=bad_idp')
             self.assertEqual(result.status_code, 302)
             self.assertEqual('/login/', result.url)
-        self.assertEqual(m.output, [self.logger_output("/login/saml/ : Bad idp param: KeyError: %s." % ("'bad_idp'",), 'info')])
+        self.assertEqual(m.output, [self.logger_output("/login/saml/ : Bad idp param: KeyError: {}.".format("'bad_idp'"), 'info')])
 
     def test_social_auth_invalid_email(self) -> None:
         """
@@ -2125,7 +2125,7 @@ class GitHubAuthBackendTest(SocialAuthBase):
             self.assertEqual(result.status_code, 302)
             self.assertEqual(result.url, "/login/")
         self.assertEqual(m.output, [self.logger_output(
-            "Social auth (%s) failed because user has no verified emails" % ('GitHub',),
+            "Social auth ({}) failed because user has no verified emails".format('GitHub'),
             "warning"
         )])
 
@@ -2454,7 +2454,7 @@ class GitHubAuthBackendTest(SocialAuthBase):
             self.assertEqual(result.status_code, 302)
             self.assertEqual(result.url, "/login/")
         self.assertEqual(m.output, [self.logger_output(
-            "Social auth (%s) failed because user has no verified emails associated with the account" % ("GitHub",),
+            "Social auth ({}) failed because user has no verified emails associated with the account".format("GitHub"),
             "warning"
         )])
 
@@ -2504,7 +2504,7 @@ class GoogleAuthBackendTest(SocialAuthBase):
             self.assertEqual(result.status_code, 302)
             self.assertEqual(result.url, "/login/")
         self.assertEqual(m.output, [self.logger_output(
-            "Social auth (%s) failed because user has no verified emails" % ("Google",),
+            "Social auth ({}) failed because user has no verified emails".format("Google"),
             "warning"
         )])
 
@@ -2946,7 +2946,7 @@ class ExternalMethodDictsTests(ZulipTestCase):
                 'id="{}_auth_button_google"'
             ]
             for name in saml_idp_names:
-                expected_button_id_strings.append('id="{}_auth_button_saml:%s"' % (name,))
+                expected_button_id_strings.append(f'id="{{}}_auth_button_saml:{name}"')
 
             result = self.client_get("/login/")
             self.assert_in_success_response([string.format("login") for string in expected_button_id_strings],
diff --git a/zerver/tests/test_bots.py b/zerver/tests/test_bots.py
index b8edc1327d..9c31e1cdec 100644
--- a/zerver/tests/test_bots.py
+++ b/zerver/tests/test_bots.py
@@ -741,7 +741,7 @@ class BotTest(ZulipTestCase, UploadSerializeMixin):
 
         # A regular user cannot reactivate a generic bot
         self.assert_num_bots_equal(0)
-        result = self.client_post("/json/users/%s/reactivate" % (bot_user.id,))
+        result = self.client_post(f"/json/users/{bot_user.id}/reactivate")
         self.assert_json_error(result, 'Must be an organization administrator')
         self.assert_num_bots_equal(0)
 
diff --git a/zerver/tests/test_decorators.py b/zerver/tests/test_decorators.py
index ca58fe8a6a..45cd481cc0 100644
--- a/zerver/tests/test_decorators.py
+++ b/zerver/tests/test_decorators.py
@@ -578,7 +578,7 @@ body:
     def test_authenticated_rest_api_view_errors(self) -> None:
         user_profile = self.example_user("hamlet")
         api_key = get_api_key(user_profile)
-        credentials = "%s:%s" % (user_profile.email, api_key)
+        credentials = f"{user_profile.email}:{api_key}"
         api_auth = 'Digest ' + base64.b64encode(credentials.encode('utf-8')).decode('utf-8')
         result = self.client_post('/api/v1/external/zendesk', {},
                                   HTTP_AUTHORIZATION=api_auth)
@@ -771,7 +771,7 @@ class ValidatorTestCase(TestCase):
             self.assertEqual(to_non_negative_int('-1'))
         with self.assertRaisesRegex(ValueError, re.escape('5 is too large (max 4)')):
             self.assertEqual(to_non_negative_int('5', max_int_size=4))
-        with self.assertRaisesRegex(ValueError, re.escape('%s is too large (max %s)' % (2**32, 2**32-1))):
+        with self.assertRaisesRegex(ValueError, re.escape(f'{2**32} is too large (max {2**32-1})')):
             self.assertEqual(to_non_negative_int(str(2**32)))
 
     def test_to_positive_or_allowed_int(self) -> None:
@@ -1055,7 +1055,7 @@ class DeactivatedRealmTest(ZulipTestCase):
         do_deactivate_realm(get_realm("zulip"))
         user_profile = self.example_user("hamlet")
         api_key = get_api_key(user_profile)
-        url = "/api/v1/external/jira?api_key=%s&stream=jira_custom" % (api_key,)
+        url = f"/api/v1/external/jira?api_key={api_key}&stream=jira_custom"
         data = self.webhook_fixture_data('jira', 'created_v2')
         result = self.client_post(url, data,
                                   content_type="application/json")
@@ -1234,7 +1234,7 @@ class InactiveUserTest(ZulipTestCase):
         do_deactivate_user(user_profile)
 
         api_key = get_api_key(user_profile)
-        url = "/api/v1/external/jira?api_key=%s&stream=jira_custom" % (api_key,)
+        url = f"/api/v1/external/jira?api_key={api_key}&stream=jira_custom"
         data = self.webhook_fixture_data('jira', 'created_v2')
         result = self.client_post(url, data,
                                   content_type="application/json")
@@ -1725,7 +1725,7 @@ class CacheTestCase(ZulipTestCase):
 
             @cachify
             def greet(first_name: str, last_name: str) -> str:
-                msg = '%s %s %s' % (greeting, first_name, last_name)
+                msg = f'{greeting} {first_name} {last_name}'
                 work_log.append(msg)
                 return msg
 
diff --git a/zerver/tests/test_email_mirror.py b/zerver/tests/test_email_mirror.py
index 3621a8dd5a..d4b8023f8d 100644
--- a/zerver/tests/test_email_mirror.py
+++ b/zerver/tests/test_email_mirror.py
@@ -339,8 +339,8 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         process_message(incoming_valid_message)
         message = most_recent_message(user_profile)
 
-        self.assertEqual(message.content, "From: %s\n%s" % (self.example_email('hamlet'),
-                                                            "TestStreamEmailMessages Body"))
+        self.assertEqual(message.content, "From: {}\n{}".format(self.example_email('hamlet'),
+                                                                "TestStreamEmailMessages Body"))
         self.assertEqual(get_display_recipient(message.recipient), stream.name)
         self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
 
@@ -364,8 +364,8 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         process_message(incoming_valid_message)
         message = most_recent_message(user_profile)
 
-        self.assertEqual(message.content, "From: %s\n%s" % ('Test Useróąę <hamlet_ę@zulip.com>',
-                                                            "TestStreamEmailMessages Body"))
+        self.assertEqual(message.content, "From: {}\n{}".format('Test Useróąę <hamlet_ę@zulip.com>',
+                                                                "TestStreamEmailMessages Body"))
         self.assertEqual(get_display_recipient(message.recipient), stream.name)
         self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
 
@@ -494,7 +494,7 @@ class TestEmailMirrorMessagesWithAttachments(ZulipTestCase):
                                                    target_realm=user_profile.realm)
 
         message = most_recent_message(user_profile)
-        self.assertEqual(message.content, "Test body\n[%s](https://test_url)" % (utf8_filename,))
+        self.assertEqual(message.content, f"Test body\n[{utf8_filename}](https://test_url)")
 
     def test_message_with_valid_nested_attachment(self) -> None:
         user_profile = self.example_user('hamlet')
diff --git a/zerver/tests/test_email_notifications.py b/zerver/tests/test_email_notifications.py
index 8e3dfef2e3..281fad1835 100644
--- a/zerver/tests/test_email_notifications.py
+++ b/zerver/tests/test_email_notifications.py
@@ -237,7 +237,7 @@ class TestMissedMessages(ZulipTestCase):
         from_email = formataddr(("Zulip missed messages", FromAddress.NOREPLY))
         self.assertEqual(len(mail.outbox), 1)
         if send_as_user:
-            from_email = '"%s" <%s>' % (othello.full_name, othello.email)
+            from_email = f'"{othello.full_name}" <{othello.email}>'
         self.assertEqual(msg.from_email, from_email)
         self.assertEqual(msg.subject, email_subject)
         self.assertEqual(len(msg.reply_to), 1)
@@ -678,9 +678,8 @@ class TestMissedMessages(ZulipTestCase):
             self.example_user('othello'), self.example_user('hamlet'),
             'Extremely personal message with a realm emoji :green_tick:!')
         realm_emoji_id = realm.get_active_emoji()['green_tick']['id']
-        realm_emoji_url = "http://zulip.testserver/user_avatars/%s/emoji/images/%s.png" % (
-            realm.id, realm_emoji_id,)
-        verify_body_include = ['<img alt=":green_tick:" src="%s" title="green tick" style="height: 20px;">' % (realm_emoji_url,)]
+        realm_emoji_url = f"http://zulip.testserver/user_avatars/{realm.id}/emoji/images/{realm_emoji_id}.png"
+        verify_body_include = [f'<img alt=":green_tick:" src="{realm_emoji_url}" title="green tick" style="height: 20px;">']
         email_subject = 'PMs with Othello, the Moor of Venice'
         self._test_cases(msg_id, verify_body_include, email_subject, send_as_user=False, verify_html_body=True)
 
diff --git a/zerver/tests/test_events.py b/zerver/tests/test_events.py
index 04ceedd56c..d37eecfb16 100644
--- a/zerver/tests/test_events.py
+++ b/zerver/tests/test_events.py
@@ -1295,7 +1295,7 @@ class EventsRegisterTest(ZulipTestCase):
         error = realm_user_add_checker('events[0]', events[0])
         self.assert_on_error(error)
         new_user_profile = get_user_by_delivery_email("test1@zulip.com", self.user_profile.realm)
-        self.assertEqual(new_user_profile.email, "user%s@zulip.testserver" % (new_user_profile.id,))
+        self.assertEqual(new_user_profile.email, f"user{new_user_profile.id}@zulip.testserver")
 
     def test_alert_words_events(self) -> None:
         alert_words_checker = self.check_events_dict([
@@ -1646,7 +1646,7 @@ class EventsRegisterTest(ZulipTestCase):
         elif property_type == (int, type(None)):
             validator = check_int
         else:
-            raise AssertionError("Unexpected property type %s" % (property_type,))
+            raise AssertionError(f"Unexpected property type {property_type}")
         schema_checker = self.check_events_dict([
             ('type', equals('realm')),
             ('op', equals('update')),
@@ -1655,7 +1655,7 @@ class EventsRegisterTest(ZulipTestCase):
         ])
 
         if vals is None:
-            raise AssertionError('No test created for %s' % (name,))
+            raise AssertionError(f'No test created for {name}')
         do_set_realm_property(self.user_profile.realm, name, vals[0])
         for val in vals[1:]:
             state_change_expected = True
@@ -1903,7 +1903,7 @@ class EventsRegisterTest(ZulipTestCase):
         elif property_type is int:
             validator = check_int
         else:
-            raise AssertionError("Unexpected property type %s" % (property_type,))
+            raise AssertionError(f"Unexpected property type {property_type}")
 
         num_events = 1
         if setting_name == "timezone":
@@ -1915,7 +1915,7 @@ class EventsRegisterTest(ZulipTestCase):
             else:
                 values = [False, True, False]
         if values is None:
-            raise AssertionError('No test created for %s' % (setting_name,))
+            raise AssertionError(f'No test created for {setting_name}')
 
         for value in values:
             events = self.do_test(lambda: do_set_user_display_setting(
@@ -2844,7 +2844,7 @@ class EventsRegisterTest(ZulipTestCase):
         ])
 
         events = self.do_test(
-            lambda: self.client_delete("/json/attachments/%s" % (entry.id,)),
+            lambda: self.client_delete(f"/json/attachments/{entry.id}"),
             num_events=1, state_change_expected=False)
         error = schema_checker('events[0]', events[0])
         self.assert_on_error(error)
diff --git a/zerver/tests/test_external.py b/zerver/tests/test_external.py
index 0071ff1c16..38a1886f34 100644
--- a/zerver/tests/test_external.py
+++ b/zerver/tests/test_external.py
@@ -93,7 +93,7 @@ class RateLimitTests(ZulipTestCase):
         start_time = time.time()
         for i in range(6):
             with mock.patch('time.time', return_value=(start_time + i * 0.1)):
-                result = self.send_api_message(user, "some stuff %s" % (i,))
+                result = self.send_api_message(user, f"some stuff {i}")
 
         self.assertEqual(result.status_code, 429)
         json = result.json()
@@ -122,5 +122,5 @@ class RateLimitTests(ZulipTestCase):
             self.assertEqual(result.status_code, 429)
             mock_warn.assert_called_with(
                 "Deadlock trying to incr_ratelimit for %s",
-                "RateLimitedUser:%s:api_by_user" % (user.id,),
+                f"RateLimitedUser:{user.id}:api_by_user",
             )
diff --git a/zerver/tests/test_home.py b/zerver/tests/test_home.py
index 3b546bd8e6..e5232cb5a7 100644
--- a/zerver/tests/test_home.py
+++ b/zerver/tests/test_home.py
@@ -257,7 +257,7 @@ class HomeTest(ZulipTestCase):
 
         for html_bit in html_bits:
             if html_bit not in html:
-                raise AssertionError('%s not in result' % (html_bit,))
+                raise AssertionError(f'{html_bit} not in result')
 
         page_params = self._get_page_params(result)
 
@@ -772,23 +772,23 @@ class HomeTest(ZulipTestCase):
         page_params = {"night_mode": True}
         add_realm_logo_fields(page_params, user_profile.realm)
         self.assertEqual(compute_navbar_logo_url(page_params),
-                         "/user_avatars/%s/realm/logo.png?version=2" % (user_profile.realm_id,))
+                         f"/user_avatars/{user_profile.realm_id}/realm/logo.png?version=2")
 
         page_params = {"night_mode": False}
         add_realm_logo_fields(page_params, user_profile.realm)
         self.assertEqual(compute_navbar_logo_url(page_params),
-                         "/user_avatars/%s/realm/logo.png?version=2" % (user_profile.realm_id,))
+                         f"/user_avatars/{user_profile.realm_id}/realm/logo.png?version=2")
 
         do_change_logo_source(user_profile.realm, Realm.LOGO_UPLOADED, night=True)
         page_params = {"night_mode": True}
         add_realm_logo_fields(page_params, user_profile.realm)
         self.assertEqual(compute_navbar_logo_url(page_params),
-                         "/user_avatars/%s/realm/night_logo.png?version=2" % (user_profile.realm_id,))
+                         f"/user_avatars/{user_profile.realm_id}/realm/night_logo.png?version=2")
 
         page_params = {"night_mode": False}
         add_realm_logo_fields(page_params, user_profile.realm)
         self.assertEqual(compute_navbar_logo_url(page_params),
-                         "/user_avatars/%s/realm/logo.png?version=2" % (user_profile.realm_id,))
+                         f"/user_avatars/{user_profile.realm_id}/realm/logo.png?version=2")
 
         # This configuration isn't super supported in the UI and is a
         # weird choice, but we have a test for it anyway.
@@ -796,7 +796,7 @@ class HomeTest(ZulipTestCase):
         page_params = {"night_mode": True}
         add_realm_logo_fields(page_params, user_profile.realm)
         self.assertEqual(compute_navbar_logo_url(page_params),
-                         "/user_avatars/%s/realm/night_logo.png?version=2" % (user_profile.realm_id,))
+                         f"/user_avatars/{user_profile.realm_id}/realm/night_logo.png?version=2")
 
         page_params = {"night_mode": False}
         add_realm_logo_fields(page_params, user_profile.realm)
diff --git a/zerver/tests/test_management_commands.py b/zerver/tests/test_management_commands.py
index c35d920332..cb47a55667 100644
--- a/zerver/tests/test_management_commands.py
+++ b/zerver/tests/test_management_commands.py
@@ -61,7 +61,7 @@ class TestZulipBaseCommand(ZulipTestCase):
         self.assertEqual(self.command.get_user(email, self.zulip_realm), user_profile)
         self.assertEqual(self.command.get_user(email, None), user_profile)
 
-        error_message = "The realm '%s' does not contain a user with email" % (mit_realm,)
+        error_message = f"The realm '{mit_realm}' does not contain a user with email"
         with self.assertRaisesRegex(CommandError, error_message):
             self.command.get_user(email, mit_realm)
 
@@ -106,7 +106,7 @@ class TestZulipBaseCommand(ZulipTestCase):
         user_emails = ','.join(u.delivery_email for u in expected_user_profiles)
         user_profiles = self.get_users_sorted(dict(users=user_emails), None)
         self.assertEqual(user_profiles, expected_user_profiles)
-        error_message = "The realm '%s' does not contain a user with email" % (self.zulip_realm,)
+        error_message = f"The realm '{self.zulip_realm}' does not contain a user with email"
         with self.assertRaisesRegex(CommandError, error_message):
             self.command.get_users(dict(users=user_emails), self.zulip_realm)
 
@@ -340,7 +340,7 @@ class TestRealmReactivationEmail(ZulipTestCase):
 
     def test_if_realm_not_deactivated(self) -> None:
         realm = get_realm('zulip')
-        with self.assertRaisesRegex(CommandError, "The realm %s is already active." % (realm.name,)):
+        with self.assertRaisesRegex(CommandError, f"The realm {realm.name} is already active."):
             call_command(self.COMMAND_NAME, "--realm=zulip")
 
 class TestSendToEmailMirror(ZulipTestCase):
diff --git a/zerver/tests/test_messages.py b/zerver/tests/test_messages.py
index 219ee98b7c..465e7b1949 100644
--- a/zerver/tests/test_messages.py
+++ b/zerver/tests/test_messages.py
@@ -279,7 +279,7 @@ class TopicHistoryTest(ZulipTestCase):
             'mit_stream',
             realm=get_realm('zephyr')
         )
-        endpoint = '/json/users/me/%s/topics' % (bad_stream.id,)
+        endpoint = f'/json/users/me/{bad_stream.id}/topics'
         result = self.client_get(endpoint, dict())
         self.assert_json_error(result, 'Invalid stream id')
 
@@ -288,7 +288,7 @@ class TopicHistoryTest(ZulipTestCase):
             'private_stream',
             invite_only=True
         )
-        endpoint = '/json/users/me/%s/topics' % (private_stream.id,)
+        endpoint = f'/json/users/me/{private_stream.id}/topics'
         result = self.client_get(endpoint, dict())
         self.assert_json_error(result, 'Invalid stream id')
 
@@ -3458,11 +3458,11 @@ class EditMessageTest(ZulipTestCase):
 
         messages = get_topic_messages(user_profile, old_stream, "test")
         self.assertEqual(len(messages), 1)
-        self.assertEqual(messages[0].content, "This topic was moved by @_**Iago|%s** to #**new stream>test**" % (user_profile.id,))
+        self.assertEqual(messages[0].content, f"This topic was moved by @_**Iago|{user_profile.id}** to #**new stream>test**")
 
         messages = get_topic_messages(user_profile, new_stream, "test")
         self.assertEqual(len(messages), 4)
-        self.assertEqual(messages[3].content, "This topic was moved here from #**test move stream>test** by @_**Iago|%s**" % (user_profile.id,))
+        self.assertEqual(messages[3].content, f"This topic was moved here from #**test move stream>test** by @_**Iago|{user_profile.id}**")
 
     def test_move_message_to_stream_change_later(self) -> None:
         (user_profile, old_stream, new_stream, msg_id, msg_id_later) = self.prepare_move_topics(
@@ -3478,7 +3478,7 @@ class EditMessageTest(ZulipTestCase):
         messages = get_topic_messages(user_profile, old_stream, "test")
         self.assertEqual(len(messages), 2)
         self.assertEqual(messages[0].id, msg_id)
-        self.assertEqual(messages[1].content, "This topic was moved by @_**Iago|%s** to #**new stream>test**" % (user_profile.id,))
+        self.assertEqual(messages[1].content, f"This topic was moved by @_**Iago|{user_profile.id}** to #**new stream>test**")
 
         messages = get_topic_messages(user_profile, new_stream, "test")
         self.assertEqual(len(messages), 3)
@@ -3535,11 +3535,11 @@ class EditMessageTest(ZulipTestCase):
 
         messages = get_topic_messages(user_profile, old_stream, "test")
         self.assertEqual(len(messages), 1)
-        self.assertEqual(messages[0].content, "This topic was moved by @_**Iago|%s** to #**new stream>new topic**" % (user_profile.id,))
+        self.assertEqual(messages[0].content, f"This topic was moved by @_**Iago|{user_profile.id}** to #**new stream>new topic**")
 
         messages = get_topic_messages(user_profile, new_stream, "new topic")
         self.assertEqual(len(messages), 4)
-        self.assertEqual(messages[3].content, "This topic was moved here from #**test move stream>test** by @_**Iago|%s**" % (user_profile.id,))
+        self.assertEqual(messages[3].content, f"This topic was moved here from #**test move stream>test** by @_**Iago|{user_profile.id}**")
         self.assert_json_success(result)
 
     def test_no_notify_move_message_to_stream(self) -> None:
@@ -3599,7 +3599,7 @@ class EditMessageTest(ZulipTestCase):
 
         messages = get_topic_messages(user_profile, old_stream, "test")
         self.assertEqual(len(messages), 1)
-        self.assertEqual(messages[0].content, "This topic was moved by @_**Iago|%s** to #**new stream>test**" % (user_profile.id,))
+        self.assertEqual(messages[0].content, f"This topic was moved by @_**Iago|{user_profile.id}** to #**new stream>test**")
 
         messages = get_topic_messages(user_profile, new_stream, "test")
         self.assertEqual(len(messages), 3)
@@ -4144,9 +4144,9 @@ class MessageHasKeywordsTest(ZulipTestCase):
         sample_size = 10
         realm_id = user_profile.realm_id
         dummy_files = [
-            ('zulip.txt', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt' % (realm_id,), sample_size),
-            ('temp_file.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py' % (realm_id,), sample_size),
-            ('abc.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py' % (realm_id,), sample_size)
+            ('zulip.txt', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt', sample_size),
+            ('temp_file.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py', sample_size),
+            ('abc.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py', sample_size)
         ]
 
         for file_name, path_id, size in dummy_files:
diff --git a/zerver/tests/test_middleware.py b/zerver/tests/test_middleware.py
index f62c281964..dbdf138ead 100644
--- a/zerver/tests/test_middleware.py
+++ b/zerver/tests/test_middleware.py
@@ -168,13 +168,13 @@ class OpenGraphTest(ZulipTestCase):
         decoded = response.content.decode('utf-8')
         bs = BeautifulSoup(decoded, features='lxml')
         open_graph_image = bs.select_one('meta[property="og:image"]').get('content')
-        self.assertEqual(open_graph_image, '%s%s' % (realm.uri, realm_icon))
+        self.assertEqual(open_graph_image, f'{realm.uri}{realm_icon}')
 
     def test_login_page_realm_icon_absolute_url(self) -> None:
         realm = get_realm('zulip')
         realm.icon_source = 'U'
         realm.save(update_fields=['icon_source'])
-        icon_url = "https://foo.s3.amazonaws.com/%s/realm/icon.png?version=%s" % (realm.id, 1)
+        icon_url = f"https://foo.s3.amazonaws.com/{realm.id}/realm/icon.png?version={1}"
         with patch('zerver.lib.realm_icon.upload_backend.get_realm_icon_url', return_value=icon_url):
             response = self.client_get('/login/')
         self.assertEqual(response.status_code, 200)
diff --git a/zerver/tests/test_narrow.py b/zerver/tests/test_narrow.py
index 9d59452b14..c258bc2396 100644
--- a/zerver/tests/test_narrow.py
+++ b/zerver/tests/test_narrow.py
@@ -2259,7 +2259,7 @@ class GetOldMessagesTest(ZulipTestCase):
             post_params = dict(required_args[:i] + required_args[i + 1:])
             result = self.client_get("/json/messages", post_params)
             self.assert_json_error(result,
-                                   "Missing '%s' argument" % (required_args[i][0],))
+                                   f"Missing '{required_args[i][0]}' argument")
 
     def test_get_messages_limits(self) -> None:
         """
@@ -2295,7 +2295,7 @@ class GetOldMessagesTest(ZulipTestCase):
                                    )
                 result = self.client_get("/json/messages", post_params)
                 self.assert_json_error(result,
-                                       "Bad value for '%s': %s" % (param, type))
+                                       f"Bad value for '{param}': {type}")
 
     def test_bad_narrow_type(self) -> None:
         """
@@ -2313,7 +2313,7 @@ class GetOldMessagesTest(ZulipTestCase):
             post_params = dict(other_params + [("narrow", type)])
             result = self.client_get("/json/messages", post_params)
             self.assert_json_error(result,
-                                   "Bad value for 'narrow': %s" % (type,))
+                                   f"Bad value for 'narrow': {type}")
 
     def test_bad_narrow_operator(self) -> None:
         """
@@ -2535,7 +2535,7 @@ class GetOldMessagesTest(ZulipTestCase):
         queries = [q for q in all_queries if '/* get_messages */' in q['sql']]
         self.assertEqual(len(queries), 1)
         sql = queries[0]['sql']
-        self.assertNotIn('AND message_id = %s' % (LARGER_THAN_MAX_MESSAGE_ID,), sql)
+        self.assertNotIn(f'AND message_id = {LARGER_THAN_MAX_MESSAGE_ID}', sql)
         self.assertIn('ORDER BY message_id ASC', sql)
 
         cond = 'WHERE user_profile_id = %d AND message_id >= %d' % (
@@ -2583,7 +2583,7 @@ class GetOldMessagesTest(ZulipTestCase):
         queries = [q for q in all_queries if '/* get_messages */' in q['sql']]
         self.assertEqual(len(queries), 1)
         sql = queries[0]['sql']
-        self.assertNotIn('AND message_id = %s' % (LARGER_THAN_MAX_MESSAGE_ID,), sql)
+        self.assertNotIn(f'AND message_id = {LARGER_THAN_MAX_MESSAGE_ID}', sql)
         self.assertIn('ORDER BY message_id ASC', sql)
         cond = 'WHERE user_profile_id = %d AND message_id <= %d' % (
             user_profile.id, first_unread_message_id - 1
@@ -2795,19 +2795,19 @@ recipient_id = %(recipient_id_3)s AND upper(subject) = upper(%(param_2)s))\
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND (sender_id = {othello_id} AND recipient_id = {hamlet_recipient} OR sender_id = {hamlet_id} AND recipient_id = {othello_recipient}) AND message_id = 0) AS anon_1 ORDER BY message_id ASC'
         sql = sql_template.format(**query_ids)
         self.common_check_get_messages_query({'anchor': 0, 'num_before': 0, 'num_after': 0,
-                                              'narrow': '[["pm-with", "%s"]]' % (othello_email,)},
+                                              'narrow': f'[["pm-with", "{othello_email}"]]'},
                                              sql)
 
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND (sender_id = {othello_id} AND recipient_id = {hamlet_recipient} OR sender_id = {hamlet_id} AND recipient_id = {othello_recipient}) AND message_id = 0) AS anon_1 ORDER BY message_id ASC'
         sql = sql_template.format(**query_ids)
         self.common_check_get_messages_query({'anchor': 0, 'num_before': 1, 'num_after': 0,
-                                              'narrow': '[["pm-with", "%s"]]' % (othello_email,)},
+                                              'narrow': f'[["pm-with", "{othello_email}"]]'},
                                              sql)
 
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND (sender_id = {othello_id} AND recipient_id = {hamlet_recipient} OR sender_id = {hamlet_id} AND recipient_id = {othello_recipient}) ORDER BY message_id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
         sql = sql_template.format(**query_ids)
         self.common_check_get_messages_query({'anchor': 0, 'num_before': 0, 'num_after': 9,
-                                              'narrow': '[["pm-with", "%s"]]' % (othello_email,)},
+                                              'narrow': f'[["pm-with", "{othello_email}"]]'},
                                              sql)
 
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND (flags & 2) != 0 ORDER BY message_id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
@@ -2819,7 +2819,7 @@ recipient_id = %(recipient_id_3)s AND upper(subject) = upper(%(param_2)s))\
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND sender_id = {othello_id} ORDER BY message_id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
         sql = sql_template.format(**query_ids)
         self.common_check_get_messages_query({'anchor': 0, 'num_before': 0, 'num_after': 9,
-                                              'narrow': '[["sender", "%s"]]' % (othello_email,)},
+                                              'narrow': f'[["sender", "{othello_email}"]]'},
                                              sql)
 
         sql_template = 'SELECT anon_1.message_id \nFROM (SELECT id AS message_id \nFROM zerver_message \nWHERE recipient_id = {scotland_recipient} ORDER BY zerver_message.id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
@@ -2856,7 +2856,7 @@ recipient_id = %(recipient_id_3)s AND upper(subject) = upper(%(param_2)s))\
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND sender_id = {hamlet_id} AND recipient_id = {hamlet_recipient} ORDER BY message_id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
         sql = sql_template.format(**query_ids)
         self.common_check_get_messages_query({'anchor': 0, 'num_before': 0, 'num_after': 9,
-                                              'narrow': '[["pm-with", "%s"]]' % (hamlet_email,)},
+                                              'narrow': f'[["pm-with", "{hamlet_email}"]]'},
                                              sql)
 
         sql_template = 'SELECT anon_1.message_id, anon_1.flags \nFROM (SELECT message_id, flags \nFROM zerver_usermessage JOIN zerver_message ON zerver_usermessage.message_id = zerver_message.id \nWHERE user_profile_id = {hamlet_id} AND recipient_id = {scotland_recipient} AND (flags & 2) != 0 ORDER BY message_id ASC \n LIMIT 10) AS anon_1 ORDER BY message_id ASC'
diff --git a/zerver/tests/test_push_notifications.py b/zerver/tests/test_push_notifications.py
index 457c3aec29..537169c682 100644
--- a/zerver/tests/test_push_notifications.py
+++ b/zerver/tests/test_push_notifications.py
@@ -209,7 +209,7 @@ class PushBouncerNotificationTest(BouncerTestCase):
 
         del self.API_KEYS[self.server_uuid]
 
-        credentials = "%s:%s" % ("5678-efgh", 'invalid')
+        credentials = "{}:{}".format("5678-efgh", 'invalid')
         api_auth = 'Basic ' + base64.b64encode(credentials.encode('utf-8')).decode('utf-8')
         result = self.client_post(endpoint, {'user_id': user_id,
                                              'token_kind': token_kind,
@@ -1900,20 +1900,17 @@ class TestPushNotificationsContent(ZulipTestCase):
         fixtures = [
             {
                 'name': 'realm_emoji',
-                'rendered_content': '<p>Testing <img alt=":green_tick:" class="emoji" src="/user_avatars/%s/emoji/green_tick.png" title="green tick"> realm emoji.</p>' % (
-                    realm.id,),
+                'rendered_content': f'<p>Testing <img alt=":green_tick:" class="emoji" src="/user_avatars/{realm.id}/emoji/green_tick.png" title="green tick"> realm emoji.</p>',
                 'expected_output': 'Testing :green_tick: realm emoji.',
             },
             {
                 'name': 'mentions',
-                'rendered_content': '<p>Mentioning <span class="user-mention" data-user-id="%s">@Cordelia Lear</span>.</p>' % (
-                    cordelia.id,),
+                'rendered_content': f'<p>Mentioning <span class="user-mention" data-user-id="{cordelia.id}">@Cordelia Lear</span>.</p>',
                 'expected_output': 'Mentioning @Cordelia Lear.',
             },
             {
                 'name': 'stream_names',
-                'rendered_content': '<p>Testing stream names <a class="stream" data-stream-id="%s" href="/#narrow/stream/Verona">#Verona</a>.</p>' % (
-                    stream.id,),
+                'rendered_content': f'<p>Testing stream names <a class="stream" data-stream-id="{stream.id}" href="/#narrow/stream/Verona">#Verona</a>.</p>',
                 'expected_output': 'Testing stream names #Verona.',
             },
         ]
diff --git a/zerver/tests/test_queue_worker.py b/zerver/tests/test_queue_worker.py
index 592a876a02..5fea7fdf85 100644
--- a/zerver/tests/test_queue_worker.py
+++ b/zerver/tests/test_queue_worker.py
@@ -384,7 +384,7 @@ class WorkerTest(ZulipTestCase):
                     self.assertEqual(mock_mirror_email.call_count, 4)
                     mock_warn.assert_called_with(
                         "Deadlock trying to incr_ratelimit for %s",
-                        "RateLimitedRealmMirror:%s" % (realm.string_id,),
+                        f"RateLimitedRealmMirror:{realm.string_id}",
                     )
 
     def test_email_sending_worker_retries(self) -> None:
diff --git a/zerver/tests/test_reactions.py b/zerver/tests/test_reactions.py
index 6450b51f4d..528fe087fa 100644
--- a/zerver/tests/test_reactions.py
+++ b/zerver/tests/test_reactions.py
@@ -154,7 +154,7 @@ class ReactionEmojiTest(ZulipTestCase):
             'emoji_name': 'smile'
         }
 
-        result = self.api_post(sender, '/api/v1/messages/%s/reactions' % (message_id,),
+        result = self.api_post(sender, f'/api/v1/messages/{message_id}/reactions',
                                reaction_info)
         self.assert_json_success(result)
 
@@ -279,7 +279,7 @@ class ReactionMessageIDTest(ZulipTestCase):
             'emoji_name': 'smile'
         }
 
-        result = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        result = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                reaction_info)
         self.assert_json_error(result, "Invalid message(s)")
 
@@ -305,11 +305,11 @@ class ReactionTest(ZulipTestCase):
             'emoji_name': 'smile'
         }
 
-        first = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        first = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                               reaction_info)
         self.assert_json_success(first)
 
-        second = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        second = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                reaction_info)
         self.assert_json_error(second, "Reaction already exists.")
 
@@ -333,15 +333,15 @@ class ReactionTest(ZulipTestCase):
             'emoji_name': 'smile'
         }
 
-        add = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        add = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                             reaction_info)
         self.assert_json_success(add)
 
-        first = self.api_delete(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        first = self.api_delete(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                 reaction_info)
         self.assert_json_success(first)
 
-        second = self.api_delete(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        second = self.api_delete(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                  reaction_info)
         self.assert_json_error(second, "Reaction doesn't exist.")
 
@@ -414,7 +414,7 @@ class ReactionEventTest(ZulipTestCase):
 
         events: List[Mapping[str, Any]] = []
         with tornado_redirected_to_list(events):
-            result = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+            result = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                    reaction_info)
         self.assert_json_success(result)
         self.assertEqual(len(events), 1)
@@ -452,13 +452,13 @@ class ReactionEventTest(ZulipTestCase):
             'emoji_name': 'smile'
         }
 
-        add = self.api_post(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+        add = self.api_post(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                             reaction_info)
         self.assert_json_success(add)
 
         events: List[Mapping[str, Any]] = []
         with tornado_redirected_to_list(events):
-            result = self.api_delete(reaction_sender, '/api/v1/messages/%s/reactions' % (pm_id,),
+            result = self.api_delete(reaction_sender, f'/api/v1/messages/{pm_id}/reactions',
                                      reaction_info)
         self.assert_json_success(result)
         self.assertEqual(len(events), 1)
@@ -486,7 +486,7 @@ class EmojiReactionBase(ZulipTestCase):
         if 'reaction_type' not in reaction_info:
             reaction_info['reaction_type'] = self.reaction_type
         sender = self.example_user(sender)
-        result = self.api_post(sender, '/api/v1/messages/%s/reactions' % (message_id,),
+        result = self.api_post(sender, f'/api/v1/messages/{message_id}/reactions',
                                reaction_info)
         return result
 
@@ -505,7 +505,7 @@ class EmojiReactionBase(ZulipTestCase):
         if 'reaction_type' not in reaction_info:
             reaction_info['reaction_type'] = self.reaction_type
         sender = self.example_user(sender)
-        result = self.api_delete(sender, '/api/v1/messages/%s/reactions' % (message_id,),
+        result = self.api_delete(sender, f'/api/v1/messages/{message_id}/reactions',
                                  reaction_info)
         return result
 
@@ -852,7 +852,7 @@ class RealmEmojiReactionTests(EmojiReactionBase):
         }
         sender = self.example_user("hamlet")
         message_id = 1
-        result = self.api_post(sender, '/api/v1/messages/%s/reactions' % (message_id,),
+        result = self.api_post(sender, f'/api/v1/messages/{message_id}/reactions',
                                reaction_info)
         self.assert_json_error(result, "Invalid emoji type.")
 
diff --git a/zerver/tests/test_realm.py b/zerver/tests/test_realm.py
index 6c6514c5ef..3334b27f61 100644
--- a/zerver/tests/test_realm.py
+++ b/zerver/tests/test_realm.py
@@ -347,7 +347,7 @@ class RealmTest(ZulipTestCase):
         invalid_lang = "invalid_lang"
         req = dict(default_language=ujson.dumps(invalid_lang))
         result = self.client_patch('/json/realm', req)
-        self.assert_json_error(result, "Invalid language '%s'" % (invalid_lang,))
+        self.assert_json_error(result, f"Invalid language '{invalid_lang}'")
         realm = get_realm('zulip')
         self.assertNotEqual(realm.default_language, invalid_lang)
 
@@ -405,20 +405,20 @@ class RealmTest(ZulipTestCase):
         self.assertEqual(realm.email_address_visibility, Realm.EMAIL_ADDRESS_VISIBILITY_ADMINS)
 
         edited_user_profile = get_user_profile_by_id(user_profile.id)
-        self.assertEqual(edited_user_profile.email, "user%s@zulip.testserver" % (edited_user_profile.id,))
+        self.assertEqual(edited_user_profile.email, f"user{edited_user_profile.id}@zulip.testserver")
 
         # Check normal user cannot access email
-        result = self.api_get(cordelia, "/api/v1/users/%s" % (hamlet.id,))
+        result = self.api_get(cordelia, f"/api/v1/users/{hamlet.id}")
         self.assert_json_success(result)
         self.assertEqual(result.json()['user']['email'],
-                         'user%s@zulip.testserver' % (hamlet.id,))
+                         f'user{hamlet.id}@zulip.testserver')
         self.assertEqual(result.json()['user'].get('delivery_email'), None)
 
         # Check administrator gets delivery_email with EMAIL_ADDRESS_VISIBILITY_ADMINS
-        result = self.api_get(user_profile, "/api/v1/users/%s" % (hamlet.id,))
+        result = self.api_get(user_profile, f"/api/v1/users/{hamlet.id}")
         self.assert_json_success(result)
         self.assertEqual(result.json()['user']['email'],
-                         'user%s@zulip.testserver' % (hamlet.id,))
+                         f'user{hamlet.id}@zulip.testserver')
         self.assertEqual(result.json()['user'].get('delivery_email'),
                          hamlet.delivery_email)
 
@@ -429,14 +429,14 @@ class RealmTest(ZulipTestCase):
         realm = get_realm("zulip")
         self.assertEqual(realm.email_address_visibility, Realm.EMAIL_ADDRESS_VISIBILITY_NOBODY)
         edited_user_profile = get_user_profile_by_id(user_profile.id)
-        self.assertEqual(edited_user_profile.email, "user%s@zulip.testserver" % (edited_user_profile.id,))
+        self.assertEqual(edited_user_profile.email, f"user{edited_user_profile.id}@zulip.testserver")
 
         # Check even administrator doesn't get delivery_email with
         # EMAIL_ADDRESS_VISIBILITY_NOBODY
-        result = self.api_get(user_profile, "/api/v1/users/%s" % (hamlet.id,))
+        result = self.api_get(user_profile, f"/api/v1/users/{hamlet.id}")
         self.assert_json_success(result)
         self.assertEqual(result.json()['user']['email'],
-                         'user%s@zulip.testserver' % (hamlet.id,))
+                         f'user{hamlet.id}@zulip.testserver')
         self.assertEqual(result.json()['user'].get('delivery_email'), None)
 
     def test_change_stream_creation_policy(self) -> None:
@@ -511,7 +511,7 @@ class RealmTest(ZulipTestCase):
         for name in integer_values:
             invalid_value = invalid_values.get(name)
             if invalid_value is None:
-                raise AssertionError('No test created for %s' % (name,))
+                raise AssertionError(f'No test created for {name}')
 
             self.do_test_invalid_integer_attribute_value(name, invalid_value)
 
@@ -724,7 +724,7 @@ class RealmAPITest(ZulipTestCase):
         if Realm.property_types[name] is bool:
             vals = bool_tests
         if vals is None:
-            raise AssertionError('No test created for %s' % (name,))
+            raise AssertionError(f'No test created for {name}')
 
         if name == 'video_chat_provider':
             self.set_up_db(name, vals[0][name])
diff --git a/zerver/tests/test_realm_emoji.py b/zerver/tests/test_realm_emoji.py
index d7d590d019..43430d25e2 100644
--- a/zerver/tests/test_realm_emoji.py
+++ b/zerver/tests/test_realm_emoji.py
@@ -86,7 +86,7 @@ class RealmEmojiTest(ZulipTestCase):
         file_name = str(realm_emoji.id) + '.png'
         self.assertEqual(
             str(realm_emoji),
-            '<RealmEmoji(zulip): %s green_tick False %s>' % (realm_emoji.id, file_name)
+            f'<RealmEmoji(zulip): {realm_emoji.id} green_tick False {file_name}>'
         )
 
     def test_upload_exception(self) -> None:
diff --git a/zerver/tests/test_retention.py b/zerver/tests/test_retention.py
index d3f6457a8c..28a1199230 100644
--- a/zerver/tests/test_retention.py
+++ b/zerver/tests/test_retention.py
@@ -135,9 +135,9 @@ class ArchiveMessagesTestingBase(RetentionTestingBase):
         host = user_profile.realm.host
         realm_id = get_realm("zulip").id
         dummy_files = [
-            ('zulip.txt', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt' % (realm_id,), sample_size),
-            ('temp_file.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py' % (realm_id,), sample_size),
-            ('abc.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py' % (realm_id,), sample_size)
+            ('zulip.txt', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt', sample_size),
+            ('temp_file.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py', sample_size),
+            ('abc.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py', sample_size)
         ]
 
         for file_name, path_id, size in dummy_files:
@@ -483,11 +483,11 @@ class MoveMessageToArchiveBase(RetentionTestingBase):
         sample_size = 10
         realm_id = get_realm("zulip").id
         dummy_files = [
-            ('zulip.txt', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt' % (realm_id,), sample_size),
-            ('temp_file.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py' % (realm_id,), sample_size),
-            ('abc.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py' % (realm_id,), sample_size),
-            ('hello.txt', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/hello.txt' % (realm_id,), sample_size),
-            ('new.py', '%s/31/4CBjtTLYZhk66pZrF8hnYGwc/new.py' % (realm_id,), sample_size)
+            ('zulip.txt', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/zulip.txt', sample_size),
+            ('temp_file.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/temp_file.py', sample_size),
+            ('abc.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/abc.py', sample_size),
+            ('hello.txt', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/hello.txt', sample_size),
+            ('new.py', f'{realm_id}/31/4CBjtTLYZhk66pZrF8hnYGwc/new.py', sample_size)
         ]
         user_profile = self.example_user('hamlet')
         for file_name, path_id, size in dummy_files:
diff --git a/zerver/tests/test_settings.py b/zerver/tests/test_settings.py
index bba51f2c75..e42516bd69 100644
--- a/zerver/tests/test_settings.py
+++ b/zerver/tests/test_settings.py
@@ -330,7 +330,7 @@ class ChangeSettingsTest(ZulipTestCase):
         test_value = test_changes.get(setting_name)
         # Error if a setting in UserProfile.property_types does not have test values
         if test_value is None:
-            raise AssertionError('No test created for %s' % (setting_name,))
+            raise AssertionError(f'No test created for {setting_name}')
 
         if isinstance(test_value, int):
             invalid_value: Any = 100
@@ -350,7 +350,7 @@ class ChangeSettingsTest(ZulipTestCase):
         result = self.client_patch("/json/settings/display", data)
         # the json error for multiple word setting names (ex: default_language)
         # displays as 'Invalid language'. Using setting_name.split('_') to format.
-        self.assert_json_error(result, "Invalid %s" % (setting_name,))
+        self.assert_json_error(result, f"Invalid {setting_name}")
 
         user_profile = self.example_user('hamlet')
         self.assertNotEqual(getattr(user_profile, setting_name), invalid_value)
diff --git a/zerver/tests/test_signup.py b/zerver/tests/test_signup.py
index 1fa8d5186a..ea90c5d7c8 100644
--- a/zerver/tests/test_signup.py
+++ b/zerver/tests/test_signup.py
@@ -1111,7 +1111,7 @@ class InviteUserTest(InviteUserBase):
 
 earl-test@zulip.com""", ["Denmark"]))
         for user in ("bob", "carol", "dave", "earl"):
-            self.assertTrue(find_key_by_email("%s-test@zulip.com" % (user,)))
+            self.assertTrue(find_key_by_email(f"{user}-test@zulip.com"))
         self.check_sent_emails(["bob-test@zulip.com", "carol-test@zulip.com",
                                 "dave-test@zulip.com", "earl-test@zulip.com"])
 
@@ -1926,7 +1926,7 @@ class MultiuseInviteTest(ZulipTestCase):
         result = self.client_post(invite_link, {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2158,7 +2158,7 @@ class RealmCreationTest(ZulipTestCase):
         result = self.client_post('/new/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/new/send_confirm/%s" % (email,)))
+            f"/accounts/new/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2255,7 +2255,7 @@ class RealmCreationTest(ZulipTestCase):
         result = self.client_post('/new/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/new/send_confirm/%s" % (email,)))
+            f"/accounts/new/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2296,7 +2296,7 @@ class RealmCreationTest(ZulipTestCase):
         result = self.client_post('/new/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/new/send_confirm/%s" % (email,)))
+            f"/accounts/new/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2508,7 +2508,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2540,7 +2540,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2608,7 +2608,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2635,7 +2635,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2667,7 +2667,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2698,7 +2698,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2726,7 +2726,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -2969,7 +2969,7 @@ class UserSignUpTest(InviteUserBase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3081,7 +3081,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
         # Visit the confirmation link.
@@ -3149,7 +3149,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
         # Visit the confirmation link.
@@ -3212,7 +3212,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3273,7 +3273,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3393,7 +3393,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3437,7 +3437,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3536,7 +3536,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3575,7 +3575,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
         with self.settings(
@@ -3690,7 +3690,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
 
@@ -3718,7 +3718,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"])
         self.assert_in_response("Check your email so we can get started.", result)
         # Visit the confirmation link.
@@ -3773,7 +3773,7 @@ class UserSignUpTest(InviteUserBase):
 
         self.assertEqual(result.status_code, 302)
         self.assertTrue(result["Location"].endswith(
-            "/accounts/send_confirm/%s" % (email,)))
+            f"/accounts/send_confirm/{email}"))
         result = self.client_get(result["Location"], subdomain="zephyr")
         self.assert_in_response("Check your email so we can get started.", result)
         # Visit the confirmation link.
diff --git a/zerver/tests/test_slack_message_conversion.py b/zerver/tests/test_slack_message_conversion.py
index 9c59808221..c3809c96de 100644
--- a/zerver/tests/test_slack_message_conversion.py
+++ b/zerver/tests/test_slack_message_conversion.py
@@ -74,7 +74,7 @@ class SlackMessageConversion(ZulipTestCase):
         self.assertEqual(full_name, 'John Doe')
         self.assertEqual(get_user_full_name(users[2]), 'Jane')
 
-        self.assertEqual(text, 'Hi @**%s**: How are you? #**general**' % (full_name,))
+        self.assertEqual(text, f'Hi @**{full_name}**: How are you? #**general**')
         self.assertEqual(mentioned_users, [540])
 
         # multiple mentioning
diff --git a/zerver/tests/test_subs.py b/zerver/tests/test_subs.py
index 84908abbba..a672a286a0 100644
--- a/zerver/tests/test_subs.py
+++ b/zerver/tests/test_subs.py
@@ -922,7 +922,7 @@ class StreamAdminTest(ZulipTestCase):
             "/json/users/me/subscriptions",
             {"subscriptions": ujson.dumps([{"name": deactivated_stream_name}])})
         self.assert_json_error(
-            result, "Unable to access stream (%s)." % (deactivated_stream_name,))
+            result, f"Unable to access stream ({deactivated_stream_name}).")
 
     def test_you_must_be_realm_admin(self) -> None:
         """
@@ -1804,7 +1804,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "in_home_view"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1812,7 +1812,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "desktop_notifications"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1820,7 +1820,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "audible_notifications"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1828,7 +1828,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "push_notifications"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1836,7 +1836,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "email_notifications"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1844,7 +1844,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": "bad",
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a boolean' % (property_name,))
+                               f'{property_name} is not a boolean')
 
         property_name = "wildcard_mentions_notify"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1853,7 +1853,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "stream_id": subs[0]["stream_id"]}])})
 
         self.assert_json_error(result,
-                               "%s is not a boolean" % (property_name,))
+                               f"{property_name} is not a boolean")
 
         property_name = "color"
         result = self.api_post(test_user, "/api/v1/users/me/subscriptions/properties",
@@ -1861,7 +1861,7 @@ class SubscriptionPropertiesTest(ZulipTestCase):
                                                                    "value": False,
                                                                    "stream_id": subs[0]["stream_id"]}])})
         self.assert_json_error(result,
-                               '%s is not a string' % (property_name,))
+                               f'{property_name} is not a string')
 
     def test_json_subscription_property_invalid_stream(self) -> None:
         test_user = self.example_user("hamlet")
@@ -2024,7 +2024,7 @@ class SubscriptionRestApiTest(ZulipTestCase):
         }
         result = self.api_patch(user, "/api/v1/users/me/subscriptions", request)
         self.assert_json_error(result,
-                               "Invalid stream name '%s'" % (invalid_stream_name,))
+                               f"Invalid stream name '{invalid_stream_name}'")
 
     def test_stream_name_too_long(self) -> None:
         user = self.example_user('hamlet')
@@ -2048,7 +2048,7 @@ class SubscriptionRestApiTest(ZulipTestCase):
         }
         result = self.api_patch(user, "/api/v1/users/me/subscriptions", request)
         self.assert_json_error(result,
-                               "Stream name '%s' contains NULL (0x00) characters." % (stream_name,))
+                               f"Stream name '{stream_name}' contains NULL (0x00) characters.")
 
     def test_compose_views_rollback(self) -> None:
         '''
@@ -2348,7 +2348,7 @@ class SubscriptionAPITest(ZulipTestCase):
         stream_name = "abc\000"
         result = self.common_subscribe_to_streams(self.test_user, [stream_name])
         self.assert_json_error(result,
-                               "Stream name '%s' contains NULL (0x00) characters." % (stream_name,))
+                               f"Stream name '{stream_name}' contains NULL (0x00) characters.")
 
     def test_user_settings_for_adding_streams(self) -> None:
         with mock.patch('zerver.models.UserProfile.can_create_streams', return_value=False):
@@ -2457,7 +2457,7 @@ class SubscriptionAPITest(ZulipTestCase):
         invalid_stream_name = ""
         result = self.common_subscribe_to_streams(self.test_user, [invalid_stream_name])
         self.assert_json_error(result,
-                               "Invalid stream name '%s'" % (invalid_stream_name,))
+                               f"Invalid stream name '{invalid_stream_name}'")
 
     def assert_adding_subscriptions_for_principal(self, invitee_data: Union[str, int], invitee_realm: Realm,
                                                   streams: List[str], invite_only: bool=False) -> None:
@@ -2821,7 +2821,7 @@ class SubscriptionAPITest(ZulipTestCase):
         mit_user = self.mit_user('starnine')
 
         realm = get_realm("zephyr")
-        stream_names = ["stream_%s" % (i,) for i in range(40)]
+        stream_names = [f"stream_{i}" for i in range(40)]
         streams = [
             self.make_stream(stream_name, realm=realm)
             for stream_name in stream_names]
@@ -2857,7 +2857,7 @@ class SubscriptionAPITest(ZulipTestCase):
     def test_bulk_subscribe_many(self) -> None:
 
         # Create a whole bunch of streams
-        streams = ["stream_%s" % (i,) for i in range(20)]
+        streams = [f"stream_{i}" for i in range(20)]
         for stream_name in streams:
             self.make_stream(stream_name)
 
@@ -3015,7 +3015,7 @@ class SubscriptionAPITest(ZulipTestCase):
         streams_to_remove = random_streams[:1]  # pick only one fake stream, to make checking the error message easy
         result = self.client_delete("/json/users/me/subscriptions",
                                     {"subscriptions": ujson.dumps(streams_to_remove)})
-        self.assert_json_error(result, "Stream(s) (%s) do not exist" % (random_streams[0],))
+        self.assert_json_error(result, f"Stream(s) ({random_streams[0]}) do not exist")
 
     def helper_subscriptions_exists(self, stream: str, expect_success: bool, subscribed: bool) -> None:
         """
@@ -3473,7 +3473,7 @@ class StreamIdTest(ZulipTestCase):
         user = self.example_user('hamlet')
         self.login_user(user)
         stream = gather_subscriptions(user)[0][0]
-        result = self.client_get("/json/get_stream_id?stream=%s" % (stream['name'],))
+        result = self.client_get("/json/get_stream_id?stream={}".format(stream['name']))
         self.assert_json_success(result)
         self.assertEqual(result.json()['stream_id'], stream['stream_id'])
 
@@ -3631,7 +3631,7 @@ class GetSubscribersTest(ZulipTestCase):
         (We also use this test to verify subscription notifications to
         folks who get subscribed to streams.)
         """
-        streams = ["stream_%s" % (i,) for i in range(10)]
+        streams = [f"stream_{i}" for i in range(10)]
         for stream_name in streams:
             self.make_stream(stream_name)
 
diff --git a/zerver/tests/test_thumbnail.py b/zerver/tests/test_thumbnail.py
index 593bbf144a..9b09d6b4a6 100644
--- a/zerver/tests/test_thumbnail.py
+++ b/zerver/tests/test_thumbnail.py
@@ -23,7 +23,7 @@ class ThumbnailTest(ZulipTestCase):
             url_in_result = 'smart/filters:no_upscale()%s/%s/source_type/s3'
             sharpen_filter = ''
             if size:
-                url_in_result = '/%s/%s' % (size, url_in_result)
+                url_in_result = f'/{size}/{url_in_result}'
                 sharpen_filter = ':sharpen(0.5,0.2,true)'
             hex_uri = base64.urlsafe_b64encode(uri.encode()).decode('utf-8')
             return url_in_result % (sharpen_filter, hex_uri)
@@ -48,13 +48,13 @@ class ThumbnailTest(ZulipTestCase):
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
 
         # Test full size image.
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri)
         self.assertIn(expected_part_url, result.url)
 
         # Test thumbnail size.
-        result = self.client_get("/thumbnail?url=%s&size=thumbnail" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=thumbnail")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri, '0x300')
         self.assertIn(expected_part_url, result.url)
@@ -72,7 +72,7 @@ class ThumbnailTest(ZulipTestCase):
         quoted_emoji_url = urllib.parse.quote(custom_emoji_url[1:], safe='')
 
         # Test full size custom emoji image (for emoji link in messages case).
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_emoji_url,))
+        result = self.client_get(f"/thumbnail?url={quoted_emoji_url}&size=full")
         self.assertEqual(result.status_code, 302, result)
         self.assertIn(custom_emoji_url, result.url)
 
@@ -88,7 +88,7 @@ class ThumbnailTest(ZulipTestCase):
 
         # Test with another user trying to access image using thumbor.
         self.login('iago')
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 403, result)
         self.assert_in_response("You are not authorized to view this file.", result)
 
@@ -98,13 +98,13 @@ class ThumbnailTest(ZulipTestCase):
             self.login('hamlet')
             quoted_url = urllib.parse.quote(image_url, safe='')
             encoded_url = base64.urlsafe_b64encode(image_url.encode()).decode('utf-8')
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_url,))
+            result = self.client_get(f"/thumbnail?url={quoted_url}&size=full")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/smart/filters:no_upscale()/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
 
             # Test thumbnail size.
-            result = self.client_get("/thumbnail?url=%s&size=thumbnail" % (quoted_url,))
+            result = self.client_get(f"/thumbnail?url={quoted_url}&size=thumbnail")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/0x300/smart/filters:no_upscale():sharpen(0.5,0.2,true)/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
@@ -113,23 +113,21 @@ class ThumbnailTest(ZulipTestCase):
             self.logout()
             user_profile = self.example_user("hamlet")
             result = self.api_get(user_profile,
-                                  "/thumbnail?url=%s&size=thumbnail" % (quoted_url,))
+                                  f"/thumbnail?url={quoted_url}&size=thumbnail")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/0x300/smart/filters:no_upscale():sharpen(0.5,0.2,true)/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
 
             # Test api endpoint with legacy API authentication.
             user_profile = self.example_user("hamlet")
-            result = self.client_get("/thumbnail?url=%s&size=thumbnail&api_key=%s" % (
-                quoted_url, get_api_key(user_profile)))
+            result = self.client_get(f"/thumbnail?url={quoted_url}&size=thumbnail&api_key={get_api_key(user_profile)}")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/0x300/smart/filters:no_upscale():sharpen(0.5,0.2,true)/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
 
             # Test a second logged-in user; they should also be able to access it
             user_profile = self.example_user("iago")
-            result = self.client_get("/thumbnail?url=%s&size=thumbnail&api_key=%s" % (
-                quoted_url, get_api_key(user_profile)))
+            result = self.client_get(f"/thumbnail?url={quoted_url}&size=thumbnail&api_key={get_api_key(user_profile)}")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/0x300/smart/filters:no_upscale():sharpen(0.5,0.2,true)/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
@@ -137,7 +135,7 @@ class ThumbnailTest(ZulipTestCase):
             # Test with another user trying to access image using thumbor.
             # File should be always accessible to user in case of external source
             self.login('iago')
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_url,))
+            result = self.client_get(f"/thumbnail?url={quoted_url}&size=full")
             self.assertEqual(result.status_code, 302, result)
             expected_part_url = '/smart/filters:no_upscale()/' + encoded_url + '/source_type/external'
             self.assertIn(expected_part_url, result.url)
@@ -156,7 +154,7 @@ class ThumbnailTest(ZulipTestCase):
             url_in_result = 'smart/filters:no_upscale()%s/%s/source_type/local_file'
             sharpen_filter = ''
             if size:
-                url_in_result = '/%s/%s' % (size, url_in_result)
+                url_in_result = f'/{size}/{url_in_result}'
                 sharpen_filter = ':sharpen(0.5,0.2,true)'
             hex_uri = base64.urlsafe_b64encode(uri.encode()).decode('utf-8')
             return url_in_result % (sharpen_filter, hex_uri)
@@ -177,13 +175,13 @@ class ThumbnailTest(ZulipTestCase):
         # We remove the forward slash infront of the `/user_uploads/` to match
         # bugdown behaviour.
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri)
         self.assertIn(expected_part_url, result.url)
 
         # Test thumbnail size.
-        result = self.client_get("/thumbnail?url=%s&size=thumbnail" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=thumbnail")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri, '0x300')
         self.assertIn(expected_part_url, result.url)
@@ -201,7 +199,7 @@ class ThumbnailTest(ZulipTestCase):
         # We remove the forward slash infront of the `/user_uploads/` to match
         # bugdown behaviour.
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri)
         self.assertIn(expected_part_url, result.url)
@@ -219,7 +217,7 @@ class ThumbnailTest(ZulipTestCase):
         quoted_emoji_url = urllib.parse.quote(custom_emoji_url[1:], safe='')
 
         # Test full size custom emoji image (for emoji link in messages case).
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_emoji_url,))
+        result = self.client_get(f"/thumbnail?url={quoted_emoji_url}&size=full")
         self.assertEqual(result.status_code, 302, result)
         self.assertIn(custom_emoji_url, result.url)
 
@@ -246,7 +244,7 @@ class ThumbnailTest(ZulipTestCase):
 
         # Test with another user trying to access image using thumbor.
         self.login('iago')
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 403, result)
         self.assert_in_response("You are not authorized to view this file.", result)
 
@@ -255,7 +253,7 @@ class ThumbnailTest(ZulipTestCase):
         self.login('hamlet')
         uri = '/static/images/cute/turtle.png'
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         self.assertEqual(uri, result.url)
 
@@ -275,14 +273,14 @@ class ThumbnailTest(ZulipTestCase):
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
 
         with self.settings(THUMBOR_URL=''):
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+            result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         self.assertEqual(uri, result.url)
 
         uri = 'https://www.google.com/images/srpr/logo4w.png'
         quoted_uri = urllib.parse.quote(uri, safe='')
         with self.settings(THUMBOR_URL=''):
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+            result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         base = 'https://external-content.zulipcdn.net/external_content/56c362a24201593891955ff526b3b412c0f9fcd2/68747470733a2f2f7777772e676f6f676c652e636f6d2f696d616765732f737270722f6c6f676f34772e706e67'
         self.assertEqual(base, result.url)
@@ -290,7 +288,7 @@ class ThumbnailTest(ZulipTestCase):
         uri = 'http://www.google.com/images/srpr/logo4w.png'
         quoted_uri = urllib.parse.quote(uri, safe='')
         with self.settings(THUMBOR_URL=''):
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+            result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         base = 'https://external-content.zulipcdn.net/external_content/7b6552b60c635e41e8f6daeb36d88afc4eabde79/687474703a2f2f7777772e676f6f676c652e636f6d2f696d616765732f737270722f6c6f676f34772e706e67'
         self.assertEqual(base, result.url)
@@ -298,7 +296,7 @@ class ThumbnailTest(ZulipTestCase):
         uri = '//www.google.com/images/srpr/logo4w.png'
         quoted_uri = urllib.parse.quote(uri, safe='')
         with self.settings(THUMBOR_URL=''):
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+            result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         base = 'https://external-content.zulipcdn.net/external_content/676530cf4b101d56f56cc4a37c6ef4d4fd9b0c03/2f2f7777772e676f6f676c652e636f6d2f696d616765732f737270722f6c6f676f34772e706e67'
         self.assertEqual(base, result.url)
@@ -319,7 +317,7 @@ class ThumbnailTest(ZulipTestCase):
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
         hex_uri = base64.urlsafe_b64encode(uri.encode()).decode('utf-8')
         with self.settings(THUMBOR_URL='http://test-thumborhost.com'):
-            result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+            result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         base = 'http://test-thumborhost.com/'
         self.assertEqual(base, result.url[:len(base)])
@@ -331,7 +329,7 @@ class ThumbnailTest(ZulipTestCase):
             url_in_result = 'smart/filters:no_upscale()%s/%s/source_type/local_file'
             sharpen_filter = ''
             if size:
-                url_in_result = '/%s/%s' % (size, url_in_result)
+                url_in_result = f'/{size}/{url_in_result}'
                 sharpen_filter = ':sharpen(0.5,0.2,true)'
             hex_uri = base64.urlsafe_b64encode(uri.encode()).decode('utf-8')
             return url_in_result % (sharpen_filter, hex_uri)
@@ -352,23 +350,23 @@ class ThumbnailTest(ZulipTestCase):
         # size=thumbnail should return a 0x300 sized image.
         # size=full should return the original resolution image.
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
-        result = self.client_get("/thumbnail?url=%s&size=thumbnail" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=thumbnail")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri, '0x300')
         self.assertIn(expected_part_url, result.url)
 
-        result = self.client_get("/thumbnail?url=%s&size=full" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
         expected_part_url = get_file_path_urlpart(uri)
         self.assertIn(expected_part_url, result.url)
 
         # Test with size supplied as a query parameter where size is anything
         # else than 'full' or 'thumbnail'. Result should be an error message.
-        result = self.client_get("/thumbnail?url=%s&size=480x360" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}&size=480x360")
         self.assertEqual(result.status_code, 403, result)
         self.assert_in_response("Invalid size.", result)
 
         # Test with no size param supplied. In this case as well we show an
         # error message.
-        result = self.client_get("/thumbnail?url=%s" % (quoted_uri,))
+        result = self.client_get(f"/thumbnail?url={quoted_uri}")
         self.assertEqual(result.status_code, 400, "Missing 'size' argument")
diff --git a/zerver/tests/test_upload.py b/zerver/tests/test_upload.py
index 95a31b919b..213dd9f722 100644
--- a/zerver/tests/test_upload.py
+++ b/zerver/tests/test_upload.py
@@ -295,8 +295,7 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
         '''
         hamlet = self.example_user("hamlet")
         self.login_user(hamlet)
-        response = self.client_get("http://localhost:9991/user_uploads/%s/ff/gg/abc.py" % (
-            hamlet.realm_id,))
+        response = self.client_get(f"http://localhost:9991/user_uploads/{hamlet.realm_id}/ff/gg/abc.py")
         self.assertEqual(response.status_code, 404)
         self.assert_in_response('File not found.', response)
 
@@ -335,8 +334,7 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
     def test_attachment_url_without_upload(self) -> None:
         hamlet = self.example_user("hamlet")
         self.login_user(hamlet)
-        body = "Test message ...[zulip.txt](http://localhost:9991/user_uploads/%s/64/fake_path_id.txt)" % (
-            hamlet.realm_id,)
+        body = f"Test message ...[zulip.txt](http://localhost:9991/user_uploads/{hamlet.realm_id}/64/fake_path_id.txt)"
         self.send_stream_message(self.example_user("hamlet"), "Denmark", body, "test")
         self.assertFalse(Attachment.objects.filter(path_id = "1/64/fake_path_id.txt").exists())
 
@@ -686,7 +684,7 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
     def test_multiple_message_attachment_file_download(self) -> None:
         hamlet = self.example_user("hamlet")
         for i in range(0, 5):
-            stream_name = "test-subscribe %s" % (i,)
+            stream_name = f"test-subscribe {i}"
             self.make_stream(stream_name, realm=hamlet.realm, invite_only=True, history_public_to_subscribers=True)
             self.subscribe(hamlet, stream_name)
 
@@ -698,7 +696,7 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
         fp_path_id = re.sub('/user_uploads/', '', uri)
         for i in range(20):
             body = f"First message ...[zulip.txt](http://{hamlet.realm.host}/user_uploads/" + fp_path_id + ")"
-            self.send_stream_message(self.example_user("hamlet"), "test-subscribe %s" % (i % 5,), body, "test")
+            self.send_stream_message(self.example_user("hamlet"), f"test-subscribe {i % 5}", body, "test")
         self.logout()
 
         user = self.example_user("aaron")
@@ -940,7 +938,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia)) + '&foo=bar'))
 
-        response = self.client_get("/avatar/%s?foo=bar" % (cordelia.id,))
+        response = self.client_get(f"/avatar/{cordelia.id}?foo=bar")
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia)) + '&foo=bar'))
 
@@ -954,7 +952,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia)) + '&foo=bar'))
 
-        response = self.api_get(hamlet, "/avatar/%s?foo=bar" % (cordelia.id,))
+        response = self.api_get(hamlet, f"/avatar/{cordelia.id}?foo=bar")
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia)) + '&foo=bar'))
 
@@ -964,7 +962,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         self.assertTrue(redirect_url.endswith(str(avatar_url(cross_realm_bot)) + '&foo=bar'))
 
         # Test cross_realm_bot avatar access using id.
-        response = self.api_get(hamlet, "/avatar/%s?foo=bar" % (cross_realm_bot.id,))
+        response = self.api_get(hamlet, f"/avatar/{cross_realm_bot.id}?foo=bar")
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cross_realm_bot)) + '&foo=bar'))
 
@@ -985,7 +983,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia, True)) + '&foo=bar'))
 
-        response = self.client_get("/avatar/%s/medium?foo=bar" % (cordelia.id,))
+        response = self.client_get(f"/avatar/{cordelia.id}/medium?foo=bar")
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia, True)) + '&foo=bar'))
 
@@ -996,7 +994,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia, True)) + '&foo=bar'))
 
-        response = self.api_get(hamlet, "/avatar/%s/medium?foo=bar" % (cordelia.id,))
+        response = self.api_get(hamlet, f"/avatar/{cordelia.id}/medium?foo=bar")
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(str(avatar_url(cordelia, True)) + '&foo=bar'))
 
@@ -1269,7 +1267,7 @@ class RealmIconTest(UploadSerializeMixin, ZulipTestCase):
             realm = get_realm('zulip')
             self.assert_json_success(result)
             self.assertIn("icon_url", result.json())
-            base = '/user_avatars/%s/realm/icon.png' % (realm.id,)
+            base = f'/user_avatars/{realm.id}/realm/icon.png'
             url = result.json()['icon_url']
             self.assertEqual(base, url[:len(base)])
 
@@ -1381,7 +1379,7 @@ class RealmLogoTest(UploadSerializeMixin, ZulipTestCase):
         response = self.client_get("/json/realm/logo", {'night': ujson.dumps(self.night)})
         redirect_url = response['Location']
         self.assertEqual(redirect_url, get_realm_logo_url(realm, self.night) +
-                         '&night=%s' % (str(self.night).lower(),))
+                         f'&night={str(self.night).lower()}')
 
     def test_get_realm_logo(self) -> None:
         self.login('hamlet')
@@ -1390,7 +1388,7 @@ class RealmLogoTest(UploadSerializeMixin, ZulipTestCase):
         response = self.client_get("/json/realm/logo", {'night': ujson.dumps(self.night)})
         redirect_url = response['Location']
         self.assertTrue(redirect_url.endswith(get_realm_logo_url(realm, self.night) +
-                                              '&night=%s' % (str(self.night).lower(),)))
+                                              f'&night={str(self.night).lower()}'))
 
     def test_valid_logos(self) -> None:
         """
@@ -1796,12 +1794,12 @@ class S3Test(ZulipTestCase):
         image_file = get_test_image_file("img.png")
         zerver.lib.upload.upload_backend.upload_realm_logo_image(image_file, user_profile, night)
 
-        original_path_id = os.path.join(str(user_profile.realm.id), "realm", "%s.original" % (file_name,))
+        original_path_id = os.path.join(str(user_profile.realm.id), "realm", f"{file_name}.original")
         original_key = bucket.Object(original_path_id)
         image_file.seek(0)
         self.assertEqual(image_file.read(), original_key.get()['Body'].read())
 
-        resized_path_id = os.path.join(str(user_profile.realm.id), "realm", "%s.png" % (file_name,))
+        resized_path_id = os.path.join(str(user_profile.realm.id), "realm", f"{file_name}.png")
         resized_data = bucket.Object(resized_path_id).get()['Body'].read()
         resized_image = Image.open(io.BytesIO(resized_data)).size
         self.assertEqual(resized_image, (DEFAULT_AVATAR_SIZE, DEFAULT_AVATAR_SIZE))
diff --git a/zerver/tests/test_urls.py b/zerver/tests/test_urls.py
index 287ff70251..60b4555fa0 100644
--- a/zerver/tests/test_urls.py
+++ b/zerver/tests/test_urls.py
@@ -113,7 +113,7 @@ class URLResolutionTest(TestCase):
 
     def check_function_exists(self, module_name: str, view: str) -> None:
         module = importlib.import_module(module_name)
-        self.assertTrue(hasattr(module, view), "View %s.%s does not exist" % (module_name, view))
+        self.assertTrue(hasattr(module, view), f"View {module_name}.{view} does not exist")
 
     # Tests that all views in urls.v1_api_and_json_patterns exist
     def test_rest_api_url_resolution(self) -> None:
diff --git a/zerver/tests/test_users.py b/zerver/tests/test_users.py
index 8d10855b0b..432f41e614 100644
--- a/zerver/tests/test_users.py
+++ b/zerver/tests/test_users.py
@@ -57,7 +57,7 @@ def find_dict(lst: Iterable[Dict[K, V]], k: K, v: V) -> Dict[K, V]:
     for dct in lst:
         if dct[k] == v:
             return dct
-    raise AssertionError('Cannot find element in list where key %s == %s' % (k, v))
+    raise AssertionError(f'Cannot find element in list where key {k} == {v}')
 
 class PermissionTest(ZulipTestCase):
     def test_role_setters(self) -> None:
@@ -263,7 +263,7 @@ class PermissionTest(ZulipTestCase):
         self.assert_json_success(result)
         members = result.json()['members']
         hamlet = find_dict(members, 'user_id', user.id)
-        self.assertEqual(hamlet['email'], "user%s@zulip.testserver" % (user.id,))
+        self.assertEqual(hamlet['email'], f"user{user.id}@zulip.testserver")
         # Note that the Gravatar URL should still be computed from the
         # `delivery_email`; otherwise, we won't be able to serve the
         # user's Gravatar.
@@ -291,7 +291,7 @@ class PermissionTest(ZulipTestCase):
         self.assert_json_success(result)
         members = result.json()['members']
         hamlet = find_dict(members, 'user_id', user.id)
-        self.assertEqual(hamlet['email'], "user%s@zulip.testserver" % (user.id,))
+        self.assertEqual(hamlet['email'], f"user{user.id}@zulip.testserver")
         self.assertEqual(hamlet['avatar_url'], get_gravatar_url(user.email, 1))
         self.assertEqual(hamlet['delivery_email'], self.example_email("hamlet"))
 
diff --git a/zerver/tornado/autoreload.py b/zerver/tornado/autoreload.py
index 64ffd60a0b..a9d7a98e54 100644
--- a/zerver/tornado/autoreload.py
+++ b/zerver/tornado/autoreload.py
@@ -189,7 +189,7 @@ def _check_file(modify_times, module, path):
     try:
         importlib.reload(module)
     except Exception:
-        gen_log.error("Error importing %s, not reloading" % (path,))
+        gen_log.error(f"Error importing {path}, not reloading")
         traceback.print_exc()
         return False
     return True
diff --git a/zerver/tornado/event_queue.py b/zerver/tornado/event_queue.py
index 9cc7b1b2d2..7e537d8ead 100644
--- a/zerver/tornado/event_queue.py
+++ b/zerver/tornado/event_queue.py
@@ -117,7 +117,7 @@ class ClientDescriptor:
                     client_type_name=self.client_type_name)
 
     def __repr__(self) -> str:
-        return "ClientDescriptor<%s>" % (self.event_queue.id,)
+        return f"ClientDescriptor<{self.event_queue.id}>"
 
     @classmethod
     def from_dict(cls, d: MutableMapping[str, Any]) -> 'ClientDescriptor':
@@ -161,7 +161,7 @@ class ClientDescriptor:
 
     def finish_current_handler(self) -> bool:
         if self.current_handler_id is not None:
-            err_msg = "Got error finishing handler for queue %s" % (self.event_queue.id,)
+            err_msg = f"Got error finishing handler for queue {self.event_queue.id}"
             try:
                 finish_handler(self.current_handler_id, self.event_queue.id,
                                self.event_queue.contents(), self.apply_markdown)
@@ -232,8 +232,8 @@ def compute_full_event_type(event: Mapping[str, Any]) -> str:
     if event["type"] == "update_message_flags":
         if event["all"]:
             # Put the "all" case in its own category
-            return "all_flags/%s/%s" % (event["flag"], event["operation"])
-        return "flags/%s/%s" % (event["operation"], event["flag"])
+            return "all_flags/{}/{}".format(event["flag"], event["operation"])
+        return "flags/{}/{}".format(event["operation"], event["flag"])
     return event["type"]
 
 class EventQueue:
@@ -561,10 +561,10 @@ def fetch_events(query: Mapping[str, Any]) -> Dict[str, Any]:
             if orig_queue_id is None:
                 response['queue_id'] = queue_id
             if len(response["events"]) == 1:
-                extra_log_data = "[%s/%s/%s]" % (queue_id, len(response["events"]),
-                                                 response["events"][0]["type"])
+                extra_log_data = "[{}/{}/{}]".format(queue_id, len(response["events"]),
+                                                     response["events"][0]["type"])
             else:
-                extra_log_data = "[%s/%s]" % (queue_id, len(response["events"]))
+                extra_log_data = "[{}/{}]".format(queue_id, len(response["events"]))
             if was_connected:
                 extra_log_data += " [was connected]"
             return dict(type="response", response=response, extra_log_data=extra_log_data)
diff --git a/zerver/tornado/handlers.py b/zerver/tornado/handlers.py
index f71322f749..e1af1f8a5c 100644
--- a/zerver/tornado/handlers.py
+++ b/zerver/tornado/handlers.py
@@ -35,11 +35,11 @@ def clear_handler_by_id(handler_id: int) -> None:
     del handlers[handler_id]
 
 def handler_stats_string() -> str:
-    return "%s handlers, latest ID %s" % (len(handlers), current_handler_id)
+    return f"{len(handlers)} handlers, latest ID {current_handler_id}"
 
 def finish_handler(handler_id: int, event_queue_id: str,
                    contents: List[Dict[str, Any]], apply_markdown: bool) -> None:
-    err_msg = "Got error finishing handler for queue %s" % (event_queue_id,)
+    err_msg = f"Got error finishing handler for queue {event_queue_id}"
     try:
         # We call async_request_timer_restart here in case we are
         # being finished without any events (because another
@@ -48,9 +48,9 @@ def finish_handler(handler_id: int, event_queue_id: str,
         request = handler._request
         async_request_timer_restart(request)
         if len(contents) != 1:
-            request._log_data['extra'] = "[%s/1]" % (event_queue_id,)
+            request._log_data['extra'] = f"[{event_queue_id}/1]"
         else:
-            request._log_data['extra'] = "[%s/1/%s]" % (event_queue_id, contents[0]["type"])
+            request._log_data['extra'] = "[{}/1/{}]".format(event_queue_id, contents[0]["type"])
 
         handler.zulip_finish(dict(result='success', msg='',
                                   events=contents,
@@ -82,7 +82,7 @@ class AsyncDjangoHandler(tornado.web.RequestHandler, base.BaseHandler):
 
     def __repr__(self) -> str:
         descriptor = get_descriptor_by_handler_id(self.handler_id)
-        return "AsyncDjangoHandler<%s, %s>" % (self.handler_id, descriptor)
+        return f"AsyncDjangoHandler<{self.handler_id}, {descriptor}>"
 
     def convert_tornado_request_to_django_request(self) -> HttpRequest:
         # This takes the WSGI environment that Tornado received (which
diff --git a/zerver/tornado/views.py b/zerver/tornado/views.py
index 701ffe02df..5e61ea6f00 100644
--- a/zerver/tornado/views.py
+++ b/zerver/tornado/views.py
@@ -29,7 +29,7 @@ def cleanup_event_queue(request: HttpRequest, user_profile: UserProfile,
         raise BadEventQueueIdError(queue_id)
     if user_profile.id != client.user_profile_id:
         return json_error(_("You are not authorized to access this queue"))
-    request._log_data['extra'] = "[%s]" % (queue_id,)
+    request._log_data['extra'] = f"[{queue_id}]"
     client.cleanup()
     return json_success()
 
diff --git a/zerver/views/auth.py b/zerver/views/auth.py
index aaa2f8ffd6..2b07424737 100644
--- a/zerver/views/auth.py
+++ b/zerver/views/auth.py
@@ -402,7 +402,7 @@ def remote_user_jwt(request: HttpRequest) -> HttpResponse:
     if email_domain is None:
         raise JsonableError(_("No organization specified in JSON web token claims"))
 
-    email = "%s@%s" % (remote_user, email_domain)
+    email = f"{remote_user}@{email_domain}"
 
     try:
         realm = get_realm(subdomain)
diff --git a/zerver/views/development/registration.py b/zerver/views/development/registration.py
index 5c188f16d5..ee8a8047ba 100644
--- a/zerver/views/development/registration.py
+++ b/zerver/views/development/registration.py
@@ -29,7 +29,7 @@ def register_development_user(request: HttpRequest) -> HttpResponse:
         request.META['HTTP_HOST'] = settings.REALM_HOSTS['zulip']
     count = UserProfile.objects.count()
     name = 'user-%d' % (count,)
-    email = '%s@zulip.com' % (name,)
+    email = f'{name}@zulip.com'
     prereg = create_preregistration_user(email, request, realm_creation=False,
                                          password_required=False)
     activation_url = create_confirmation_link(prereg, request.get_host(),
@@ -44,7 +44,7 @@ def register_development_user(request: HttpRequest) -> HttpResponse:
 def register_development_realm(request: HttpRequest) -> HttpResponse:
     count = UserProfile.objects.count()
     name = 'user-%d' % (count,)
-    email = '%s@zulip.com' % (name,)
+    email = f'{name}@zulip.com'
     realm_name = 'realm-%d' % (count,)
     prereg = create_preregistration_user(email, request, realm_creation=True,
                                          password_required=False)
diff --git a/zerver/views/documentation.py b/zerver/views/documentation.py
index 452b84784b..f26c49b4bd 100644
--- a/zerver/views/documentation.py
+++ b/zerver/views/documentation.py
@@ -110,11 +110,11 @@ class MarkdownDirectoryView(ApiURLView):
             # Strip the header and then use the first line to get the article title
             article_title = first_line.lstrip("#").strip()
             if context["not_index_page"]:
-                context["OPEN_GRAPH_TITLE"] = "%s (%s)" % (article_title, title_base)
+                context["OPEN_GRAPH_TITLE"] = f"{article_title} ({title_base})"
             else:
                 context["OPEN_GRAPH_TITLE"] = title_base
             self.request.placeholder_open_graph_description = (
-                "REPLACMENT_OPEN_GRAPH_DESCRIPTION_%s" % (int(2**24 * random.random()),))
+                f"REPLACMENT_OPEN_GRAPH_DESCRIPTION_{int(2**24 * random.random())}")
             context["OPEN_GRAPH_DESCRIPTION"] = self.request.placeholder_open_graph_description
 
         context["sidebar_index"] = sidebar_index
diff --git a/zerver/views/home.py b/zerver/views/home.py
index ada3020002..749da6c1d8 100644
--- a/zerver/views/home.py
+++ b/zerver/views/home.py
@@ -301,7 +301,7 @@ def home_real(request: HttpRequest) -> HttpResponse:
         if user_profile.realm.plan_type == Realm.LIMITED:
             show_plans = True
 
-    request._log_data['extra'] = "[%s]" % (register_ret["queue_id"],)
+    request._log_data['extra'] = "[{}]".format(register_ret["queue_id"])
 
     page_params['translation_data'] = {}
     if request_language != 'en':
diff --git a/zerver/views/messages.py b/zerver/views/messages.py
index 3295c2c541..f1133dc880 100644
--- a/zerver/views/messages.py
+++ b/zerver/views/messages.py
@@ -95,7 +95,7 @@ TS_STOP = "</ts-match>"
 def ts_locs_array(
     config: ColumnElement, text: ColumnElement, tsquery: ColumnElement
 ) -> ColumnElement:
-    options = "HighlightAll = TRUE, StartSel = %s, StopSel = %s" % (TS_START, TS_STOP)
+    options = f"HighlightAll = TRUE, StartSel = {TS_START}, StopSel = {TS_STOP}"
     delimited = func.ts_headline(config, text, tsquery, options)
     parts = func.unnest(func.string_to_array(delimited, TS_START)).alias()
     part = column(parts.name)
@@ -255,7 +255,7 @@ class NarrowBuilder:
             base_stream_name = m.group(1)
 
             matching_streams = get_active_streams(self.user_profile.realm).filter(
-                name__iregex=r'^(un)*%s(\.d)*$' % (self._pg_re_escape(base_stream_name),))
+                name__iregex=fr'^(un)*{self._pg_re_escape(base_stream_name)}(\.d)*$')
             recipient_ids = [matching_stream.recipient_id for matching_stream in matching_streams]
             cond = column("recipient_id").in_(recipient_ids)
             return query.where(maybe_negate(cond))
@@ -851,7 +851,7 @@ def get_messages_backend(request: HttpRequest, user_profile: UserProfile,
                 verbose_operators.append("is:" + term['operand'])
             else:
                 verbose_operators.append(term['operator'])
-        request._log_data['extra'] = "[%s]" % (",".join(verbose_operators),)
+        request._log_data['extra'] = "[{}]".format(",".join(verbose_operators))
 
     sa_conn = get_sqlalchemy_connection()
 
@@ -1127,7 +1127,7 @@ def update_message_flags(request: HttpRequest, user_profile: UserProfile,
     count = do_update_message_flags(user_profile, request.client, operation, flag, messages)
 
     target_count_str = str(len(messages))
-    log_data_str = "[%s %s/%s] actually %s" % (operation, flag, target_count_str, count)
+    log_data_str = f"[{operation} {flag}/{target_count_str}] actually {count}"
     request._log_data["extra"] = log_data_str
 
     return json_success({'result': 'success',
@@ -1138,7 +1138,7 @@ def update_message_flags(request: HttpRequest, user_profile: UserProfile,
 def mark_all_as_read(request: HttpRequest, user_profile: UserProfile) -> HttpResponse:
     count = do_mark_all_as_read(user_profile, request.client)
 
-    log_data_str = "[%s updated]" % (count,)
+    log_data_str = f"[{count} updated]"
     request._log_data["extra"] = log_data_str
 
     return json_success({'result': 'success',
@@ -1151,7 +1151,7 @@ def mark_stream_as_read(request: HttpRequest,
     stream, recipient, sub = access_stream_by_id(user_profile, stream_id)
     count = do_mark_stream_messages_as_read(user_profile, request.client, stream)
 
-    log_data_str = "[%s updated]" % (count,)
+    log_data_str = f"[{count} updated]"
     request._log_data["extra"] = log_data_str
 
     return json_success({'result': 'success',
@@ -1176,7 +1176,7 @@ def mark_topic_as_read(request: HttpRequest,
 
     count = do_mark_stream_messages_as_read(user_profile, request.client, stream, topic_name)
 
-    log_data_str = "[%s updated]" % (count,)
+    log_data_str = f"[{count} updated]"
     request._log_data["extra"] = log_data_str
 
     return json_success({'result': 'success',
@@ -1616,7 +1616,7 @@ def update_message_backend(request: HttpRequest, user_profile: UserMessage,
                                        mention_user_ids, mention_data)
 
     # Include the number of messages changed in the logs
-    request._log_data['extra'] = "[%s]" % (number_changed,)
+    request._log_data['extra'] = f"[{number_changed}]"
     if links_for_embed:
         event_data = {
             'message_id': message.id,
diff --git a/zerver/views/pointer.py b/zerver/views/pointer.py
index c01ba35489..924382098d 100644
--- a/zerver/views/pointer.py
+++ b/zerver/views/pointer.py
@@ -19,7 +19,7 @@ def update_pointer_backend(request: HttpRequest, user_profile: UserProfile,
     if get_usermessage_by_message_id(user_profile, pointer) is None:
         raise JsonableError(_("Invalid message ID"))
 
-    request._log_data["extra"] = "[%s]" % (pointer,)
+    request._log_data["extra"] = f"[{pointer}]"
     update_flags = (request.client.name.lower() in ['android', "zulipandroid"])
     do_update_pointer(user_profile, request.client, pointer, update_flags=update_flags)
 
diff --git a/zerver/views/report.py b/zerver/views/report.py
index 3ce197d914..981f82c102 100644
--- a/zerver/views/report.py
+++ b/zerver/views/report.py
@@ -51,11 +51,11 @@ def report_send_times(request: HttpRequest, user_profile: UserProfile,
         % (time, received_str, displayed_str, locally_echoed, rendered_content_disparity)
 
     base_key = statsd_key(user_profile.realm.string_id, clean_periods=True)
-    statsd.timing("endtoend.send_time.%s" % (base_key,), time)
+    statsd.timing(f"endtoend.send_time.{base_key}", time)
     if received > 0:
-        statsd.timing("endtoend.receive_time.%s" % (base_key,), received)
+        statsd.timing(f"endtoend.receive_time.{base_key}", received)
     if displayed > 0:
-        statsd.timing("endtoend.displayed_time.%s" % (base_key,), displayed)
+        statsd.timing(f"endtoend.displayed_time.{base_key}", displayed)
     if locally_echoed:
         statsd.incr('locally_echoed')
     if rendered_content_disparity:
@@ -68,11 +68,11 @@ def report_narrow_times(request: HttpRequest, user_profile: UserProfile,
                         initial_core: int=REQ(converter=to_non_negative_int),
                         initial_free: int=REQ(converter=to_non_negative_int),
                         network: int=REQ(converter=to_non_negative_int)) -> HttpResponse:
-    request._log_data["extra"] = "[%sms/%sms/%sms]" % (initial_core, initial_free, network)
+    request._log_data["extra"] = f"[{initial_core}ms/{initial_free}ms/{network}ms]"
     base_key = statsd_key(user_profile.realm.string_id, clean_periods=True)
-    statsd.timing("narrow.initial_core.%s" % (base_key,), initial_core)
-    statsd.timing("narrow.initial_free.%s" % (base_key,), initial_free)
-    statsd.timing("narrow.network.%s" % (base_key,), network)
+    statsd.timing(f"narrow.initial_core.{base_key}", initial_core)
+    statsd.timing(f"narrow.initial_free.{base_key}", initial_free)
+    statsd.timing(f"narrow.network.{base_key}", network)
     return json_success()
 
 @human_users_only
@@ -80,10 +80,10 @@ def report_narrow_times(request: HttpRequest, user_profile: UserProfile,
 def report_unnarrow_times(request: HttpRequest, user_profile: UserProfile,
                           initial_core: int=REQ(converter=to_non_negative_int),
                           initial_free: int=REQ(converter=to_non_negative_int)) -> HttpResponse:
-    request._log_data["extra"] = "[%sms/%sms]" % (initial_core, initial_free)
+    request._log_data["extra"] = f"[{initial_core}ms/{initial_free}ms]"
     base_key = statsd_key(user_profile.realm.string_id, clean_periods=True)
-    statsd.timing("unnarrow.initial_core.%s" % (base_key,), initial_core)
-    statsd.timing("unnarrow.initial_free.%s" % (base_key,), initial_free)
+    statsd.timing(f"unnarrow.initial_core.{base_key}", initial_core)
+    statsd.timing(f"unnarrow.initial_free.{base_key}", initial_free)
     return json_success()
 
 @has_request_variables
diff --git a/zerver/views/streams.py b/zerver/views/streams.py
index b2ec544a65..4876f04f72 100644
--- a/zerver/views/streams.py
+++ b/zerver/views/streams.py
@@ -312,7 +312,7 @@ def you_were_just_subscribed_message(acting_user: UserProfile,
         {"full_name": acting_user.full_name}
     message += "\n\n"
     for stream_name in subscriptions:
-        message += "* #**%s**\n" % (stream_name,)
+        message += f"* #**{stream_name}**\n"
     return message
 
 @require_non_guest_user
@@ -443,7 +443,7 @@ def add_subscriptions_backend(
             content = content % {
                 'user_name': user_profile.full_name,
                 'user_id': user_profile.id,
-                'stream_str': ", ".join('#**%s**' % (s.name,) for s in created_streams)}
+                'stream_str': ", ".join(f'#**{s.name}**' for s in created_streams)}
 
             sender = get_system_bot(settings.NOTIFICATION_BOT)
             topic = _('new streams')
diff --git a/zerver/views/upload.py b/zerver/views/upload.py
index f7f1c06295..22ff8389b4 100644
--- a/zerver/views/upload.py
+++ b/zerver/views/upload.py
@@ -70,7 +70,7 @@ def serve_file_url_backend(request: HttpRequest, user_profile: UserProfile,
 def serve_file(request: HttpRequest, user_profile: UserProfile,
                realm_id_str: str, filename: str,
                url_only: bool=False) -> HttpResponse:
-    path_id = "%s/%s" % (realm_id_str, filename)
+    path_id = f"{realm_id_str}/{filename}"
     is_authorized = validate_attachment_request(user_profile, path_id)
 
     if is_authorized is None:
diff --git a/zerver/views/users.py b/zerver/views/users.py
index 3c7696c8c8..26b08d5e8a 100644
--- a/zerver/views/users.py
+++ b/zerver/views/users.py
@@ -268,7 +268,7 @@ def add_bot_backend(
     short_name += "-bot"
     full_name = check_full_name(full_name_raw)
     try:
-        email = '%s@%s' % (short_name, user_profile.realm.get_bot_domain())
+        email = f'{short_name}@{user_profile.realm.get_bot_domain()}'
     except InvalidFakeEmailDomain:
         return json_error(_("Can't create bots until FAKE_EMAIL_DOMAIN is correctly configured.\n"
                             "Please contact your server administrator."))
diff --git a/zerver/webhooks/beanstalk/view.py b/zerver/webhooks/beanstalk/view.py
index d70e74d363..2567bf102f 100644
--- a/zerver/webhooks/beanstalk/view.py
+++ b/zerver/webhooks/beanstalk/view.py
@@ -53,7 +53,7 @@ def beanstalk_decoder(view_func: ViewFuncT) -> ViewFuncT:
         if auth_type.lower() == "basic":
             email, api_key = base64.b64decode(encoded_value).decode('utf-8').split(":")
             email = email.replace('%40', '@')
-            credentials = "%s:%s" % (email, api_key)
+            credentials = f"{email}:{api_key}"
             encoded_credentials: str = base64.b64encode(credentials.encode('utf-8')).decode('utf8')
             request.META['HTTP_AUTHORIZATION'] = "Basic " + encoded_credentials
 
@@ -89,8 +89,8 @@ def api_beanstalk_webhook(request: HttpRequest, user_profile: UserProfile,
         revision = payload.get('revision')
         (short_commit_msg, _, _) = payload['message'].partition("\n")
 
-        subject = "svn r%s" % (revision,)
-        content = "%s pushed [revision %s](%s):\n\n> %s" % (author, revision, url, short_commit_msg)
+        subject = f"svn r{revision}"
+        content = f"{author} pushed [revision {revision}]({url}):\n\n> {short_commit_msg}"
 
     check_send_webhook_message(request, user_profile, subject, content)
     return json_success()
diff --git a/zerver/webhooks/bitbucket3/view.py b/zerver/webhooks/bitbucket3/view.py
index a6a6fab9ca..16a469ecd9 100644
--- a/zerver/webhooks/bitbucket3/view.py
+++ b/zerver/webhooks/bitbucket3/view.py
@@ -65,7 +65,7 @@ def repo_comment_handler(payload: Dict[str, Any], action: str) -> List[Dict[str,
     subject = BITBUCKET_TOPIC_TEMPLATE.format(repository_name=repo_name)
     sha = payload["commit"]
     commit_url = payload["repository"]["links"]["self"][0]["href"][:-6]  # remove the "browse" at the end
-    commit_url += "commits/%s" % (sha,)
+    commit_url += f"commits/{sha}"
     message = payload["comment"]["text"]
     if action == "deleted their comment":
         message = f"~~{message}~~"
@@ -125,7 +125,7 @@ def repo_push_branch_data(payload: Dict[str, Any], change: Dict[str, Any]) -> Di
     elif event_type == "DELETE":
         body = get_remove_branch_event_message(user_name, branch_name)
     else:
-        message = "%s.%s" % (payload["eventKey"], event_type)  # nocoverage
+        message = "{}.{}".format(payload["eventKey"], event_type)  # nocoverage
         raise UnexpectedWebhookEventType("BitBucket Server", message)
 
     subject = TOPIC_WITH_BRANCH_TEMPLATE.format(repo=repo_name, branch=branch_name)
@@ -141,7 +141,7 @@ def repo_push_tag_data(payload: Dict[str, Any], change: Dict[str, Any]) -> Dict[
     elif event_type == "DELETE":
         action = "removed"
     else:
-        message = "%s.%s" % (payload["eventKey"], event_type)  # nocoverage
+        message = "{}.{}".format(payload["eventKey"], event_type)  # nocoverage
         raise UnexpectedWebhookEventType("BitBucket Server", message)
 
     subject = BITBUCKET_TOPIC_TEMPLATE.format(repository_name=repo_name)
@@ -162,7 +162,7 @@ def repo_push_handler(payload: Dict[str, Any], branches: Optional[str]=None
         elif event_target_type == "TAG":
             data.append(repo_push_tag_data(payload, change))
         else:
-            message = "%s.%s" % (payload["eventKey"], event_target_type)  # nocoverage
+            message = "{}.{}".format(payload["eventKey"], event_target_type)  # nocoverage
             raise UnexpectedWebhookEventType("BitBucket Server", message)
     return data
 
@@ -171,7 +171,7 @@ def get_assignees_string(pr: Dict[str, Any]) -> Optional[str]:
     for reviewer in pr["reviewers"]:
         name = reviewer["user"]["name"]
         link = reviewer["user"]["links"]["self"][0]["href"]
-        reviewers.append("[%s](%s)" % (name, link))
+        reviewers.append(f"[{name}]({link})")
     if len(reviewers) == 0:
         assignees = None
     elif len(reviewers) == 1:
diff --git a/zerver/webhooks/front/view.py b/zerver/webhooks/front/view.py
index 40659f4505..819bb41b96 100644
--- a/zerver/webhooks/front/view.py
+++ b/zerver/webhooks/front/view.py
@@ -20,12 +20,12 @@ def get_message_data(payload: Dict[str, Any]) -> Tuple[str, str, str, str]:
 def get_source_name(payload: Dict[str, Any]) -> str:
     first_name = payload['source']['data']['first_name']
     last_name = payload['source']['data']['last_name']
-    return "%s %s" % (first_name, last_name)
+    return f"{first_name} {last_name}"
 
 def get_target_name(payload: Dict[str, Any]) -> str:
     first_name = payload['target']['data']['first_name']
     last_name = payload['target']['data']['last_name']
-    return "%s %s" % (first_name, last_name)
+    return f"{first_name} {last_name}"
 
 def get_inbound_message_body(payload: Dict[str, Any]) -> str:
     link, outbox, inbox, subject = get_message_data(payload)
diff --git a/zerver/webhooks/jira/tests.py b/zerver/webhooks/jira/tests.py
index 755fe20b39..435d42519b 100644
--- a/zerver/webhooks/jira/tests.py
+++ b/zerver/webhooks/jira/tests.py
@@ -10,7 +10,7 @@ class JiraHookTests(WebhookTestCase):
 
     def test_custom_stream(self) -> None:
         api_key = get_api_key(self.test_user)
-        url = "/api/v1/external/jira?api_key=%s&stream=jira_custom" % (api_key,)
+        url = f"/api/v1/external/jira?api_key={api_key}&stream=jira_custom"
         msg = self.send_json_payload(self.test_user,
                                      url,
                                      self.get_body('created_v2'),
diff --git a/zerver/webhooks/newrelic/view.py b/zerver/webhooks/newrelic/view.py
index df3f611d86..6760c38e8b 100644
--- a/zerver/webhooks/newrelic/view.py
+++ b/zerver/webhooks/newrelic/view.py
@@ -40,7 +40,7 @@ def api_newrelic_webhook(request: HttpRequest, user_profile: UserProfile,
         subject = alert['message']
         content = ALERT_TEMPLATE.format(**alert)
     elif deployment:
-        subject = "%s deploy" % (deployment['application_name'],)
+        subject = "{} deploy".format(deployment['application_name'])
         content = DEPLOY_TEMPLATE.format(**deployment)
     else:
         raise UnexpectedWebhookEventType('New Relic', 'Unknown Event Type')
diff --git a/zerver/webhooks/pivotal/view.py b/zerver/webhooks/pivotal/view.py
index 31965e3e32..3a56f4b2dd 100644
--- a/zerver/webhooks/pivotal/view.py
+++ b/zerver/webhooks/pivotal/view.py
@@ -33,7 +33,7 @@ def api_pivotal_webhook_v3(request: HttpRequest, user_profile: UserProfile) -> T
     story_id = get_text(['stories', 'story', 'id'])
     # Ugh, the URL in the XML data is not a clickable url that works for the user
     # so we try to build one that the user can actually click on
-    url = "https://www.pivotaltracker.com/s/projects/%s/stories/%s" % (project_id, story_id)
+    url = f"https://www.pivotaltracker.com/s/projects/{project_id}/stories/{story_id}"
 
     # Pivotal doesn't tell us the name of the story, but it's usually in the
     # description in quotes as the first quoted string
@@ -43,7 +43,7 @@ def api_pivotal_webhook_v3(request: HttpRequest, user_profile: UserProfile) -> T
         name = match.group(1)
     else:
         name = "Story changed"  # Failed for an unknown reason, show something
-    more_info = " [(view)](%s)." % (url,)
+    more_info = f" [(view)]({url})."
 
     if event_type == 'story_update':
         subject = name
@@ -57,15 +57,9 @@ def api_pivotal_webhook_v3(request: HttpRequest, user_profile: UserProfile) -> T
         issue_status = get_text(['stories', 'story', 'current_state'])
         estimate = get_text(['stories', 'story', 'estimate'])
         if estimate != '':
-            estimate = " worth %s story points" % (estimate,)
+            estimate = f" worth {estimate} story points"
         subject = name
-        content = "%s (%s %s%s):\n\n~~~ quote\n%s\n~~~\n\n%s" % (
-            description,
-            issue_status,
-            issue_type,
-            estimate,
-            issue_desc,
-            more_info)
+        content = f"{description} ({issue_status} {issue_type}{estimate}):\n\n~~~ quote\n{issue_desc}\n~~~\n\n{more_info}"
     return subject, content
 
 UNSUPPORTED_EVENT_TYPES = [
@@ -95,13 +89,12 @@ def api_pivotal_webhook_v5(request: HttpRequest, user_profile: UserProfile) -> T
 
     performed_by = payload.get("performed_by", {}).get("name", "")
 
-    story_info = "[%s](https://www.pivotaltracker.com/s/projects/%s): [%s](%s)" % (
-        project_name, project_id, story_name, story_url)
+    story_info = f"[{project_name}](https://www.pivotaltracker.com/s/projects/{project_id}): [{story_name}]({story_url})"
 
     changes = payload.get("changes", [])
 
     content = ""
-    subject = "#%s: %s" % (story_id, story_name)
+    subject = f"#{story_id}: {story_name}"
 
     def extract_comment(change: Dict[str, Any]) -> Optional[str]:
         if change.get("kind") == "comment":
@@ -110,52 +103,51 @@ def api_pivotal_webhook_v5(request: HttpRequest, user_profile: UserProfile) -> T
 
     if event_type == "story_update_activity":
         # Find the changed valued and build a message
-        content += "%s updated %s:\n" % (performed_by, story_info)
+        content += f"{performed_by} updated {story_info}:\n"
         for change in changes:
             old_values = change.get("original_values", {})
             new_values = change["new_values"]
 
             if "current_state" in old_values and "current_state" in new_values:
-                content += "* state changed from **%s** to **%s**\n" % (
+                content += "* state changed from **{}** to **{}**\n".format(
                     old_values["current_state"], new_values["current_state"])
             if "estimate" in old_values and "estimate" in new_values:
                 old_estimate = old_values.get("estimate", None)
                 if old_estimate is None:
                     estimate = "is now"
                 else:
-                    estimate = "changed from %s to" % (old_estimate,)
+                    estimate = f"changed from {old_estimate} to"
                 new_estimate = new_values["estimate"] if new_values["estimate"] is not None else "0"
-                content += "* estimate %s **%s points**\n" % (estimate, new_estimate)
+                content += f"* estimate {estimate} **{new_estimate} points**\n"
             if "story_type" in old_values and "story_type" in new_values:
-                content += "* type changed from **%s** to **%s**\n" % (
+                content += "* type changed from **{}** to **{}**\n".format(
                     old_values["story_type"], new_values["story_type"])
 
             comment = extract_comment(change)
             if comment is not None:
-                content += "* Comment added:\n~~~quote\n%s\n~~~\n" % (comment,)
+                content += f"* Comment added:\n~~~quote\n{comment}\n~~~\n"
 
     elif event_type == "comment_create_activity":
         for change in changes:
             comment = extract_comment(change)
             if comment is not None:
-                content += "%s added a comment to %s:\n~~~quote\n%s\n~~~" % (
-                    performed_by, story_info, comment)
+                content += f"{performed_by} added a comment to {story_info}:\n~~~quote\n{comment}\n~~~"
     elif event_type == "story_create_activity":
-        content += "%s created %s: %s\n" % (performed_by, story_type, story_info)
+        content += f"{performed_by} created {story_type}: {story_info}\n"
         for change in changes:
             new_values = change.get("new_values", {})
             if "current_state" in new_values:
-                content += "* State is **%s**\n" % (new_values["current_state"],)
+                content += "* State is **{}**\n".format(new_values["current_state"])
             if "description" in new_values:
-                content += "* Description is\n\n> %s" % (new_values["description"],)
+                content += "* Description is\n\n> {}".format(new_values["description"])
     elif event_type == "story_move_activity":
-        content = "%s moved %s" % (performed_by, story_info)
+        content = f"{performed_by} moved {story_info}"
         for change in changes:
             old_values = change.get("original_values", {})
             new_values = change["new_values"]
             if "current_state" in old_values and "current_state" in new_values:
-                content += " from **%s** to **%s**." % (old_values["current_state"],
-                                                        new_values["current_state"])
+                content += " from **{}** to **{}**.".format(old_values["current_state"],
+                                                            new_values["current_state"])
     elif event_type in UNSUPPORTED_EVENT_TYPES:
         # Known but unsupported Pivotal event types
         pass
diff --git a/zerver/webhooks/zendesk/view.py b/zerver/webhooks/zendesk/view.py
index 50a3c305cd..4292ba150d 100644
--- a/zerver/webhooks/zendesk/view.py
+++ b/zerver/webhooks/zendesk/view.py
@@ -23,6 +23,6 @@ def api_zendesk_webhook(request: HttpRequest, user_profile: UserProfile,
     ticket_id and ticket_title to create a subject. And passes with zendesk
     user's configured message to zulip.
     """
-    subject = truncate('#%s: %s' % (ticket_id, ticket_title), 60)
+    subject = truncate(f'#{ticket_id}: {ticket_title}', 60)
     check_send_webhook_message(request, user_profile, subject, message)
     return json_success()
diff --git a/zerver/worker/queue_processors.py b/zerver/worker/queue_processors.py
index 924f32e2b3..1e76dbf760 100644
--- a/zerver/worker/queue_processors.py
+++ b/zerver/worker/queue_processors.py
@@ -150,7 +150,7 @@ class QueueProcessingWorker(ABC):
 
         os.makedirs(settings.QUEUE_STATS_DIR, exist_ok=True)
 
-        fname = '%s.stats' % (self.queue_name,)
+        fname = f'{self.queue_name}.stats'
         fn = os.path.join(settings.QUEUE_STATS_DIR, fname)
         with lockfile(fn + '.lock'):
             tmp_fn = fn + '.tmp'
@@ -204,9 +204,9 @@ class QueueProcessingWorker(ABC):
         self._log_problem()
         if not os.path.exists(settings.QUEUE_ERROR_DIR):
             os.mkdir(settings.QUEUE_ERROR_DIR)  # nocoverage
-        fname = '%s.errors' % (self.queue_name,)
+        fname = f'{self.queue_name}.errors'
         fn = os.path.join(settings.QUEUE_ERROR_DIR, fname)
-        line = '%s\t%s\n' % (time.asctime(), ujson.dumps(events))
+        line = f'{time.asctime()}\t{ujson.dumps(events)}\n'
         lock_fn = fn + '.lock'
         with lockfile(lock_fn):
             with open(fn, 'ab') as f:
@@ -214,7 +214,7 @@ class QueueProcessingWorker(ABC):
         check_and_send_restart_signal()
 
     def _log_problem(self) -> None:
-        logging.exception("Problem handling data on queue %s" % (self.queue_name,))
+        logging.exception(f"Problem handling data on queue {self.queue_name}")
 
     def setup(self) -> None:
         self.q = SimpleQueueClient()
@@ -685,8 +685,7 @@ class DeferredWorker(QueueProcessingWorker):
 
             # Send a private message notification letting the user who
             # triggered the export know the export finished.
-            content = "Your data export is complete and has been uploaded here:\n\n%s" % (
-                public_url,)
+            content = f"Your data export is complete and has been uploaded here:\n\n{public_url}"
             internal_send_private_message(
                 realm=user_profile.realm,
                 sender=get_system_bot(settings.NOTIFICATION_BOT),
diff --git a/zilencer/management/commands/add_new_realm.py b/zilencer/management/commands/add_new_realm.py
index 7570e0c75e..a78723114d 100644
--- a/zilencer/management/commands/add_new_realm.py
+++ b/zilencer/management/commands/add_new_realm.py
@@ -17,7 +17,7 @@ class Command(ZulipBaseCommand):
 
         name = '%02d-user' % (
             UserProfile.objects.filter(email__contains='user@').count(),)
-        user = do_create_user('%s@%s.zulip.com' % (name, string_id),
+        user = do_create_user(f'{name}@{string_id}.zulip.com',
                               'password', realm, name, name, role=UserProfile.ROLE_REALM_ADMINISTRATOR)
         bulk_add_subscriptions([realm.signup_notifications_stream], [user])
 
diff --git a/zilencer/management/commands/add_new_user.py b/zilencer/management/commands/add_new_user.py
index 1ed1e179af..e6fdfce4af 100644
--- a/zilencer/management/commands/add_new_user.py
+++ b/zilencer/management/commands/add_new_user.py
@@ -30,4 +30,4 @@ and will otherwise fall back to the zulip realm."""
             domain = realm.string_id + '.zulip.com'
 
         name = '%02d-user' % (UserProfile.objects.filter(email__contains='user@').count(),)
-        do_create_user('%s@%s' % (name, domain), 'password', valid_realm, name, name)
+        do_create_user(f'{name}@{domain}', 'password', valid_realm, name, name)
diff --git a/zilencer/management/commands/populate_db.py b/zilencer/management/commands/populate_db.py
index e1b5b921d2..f74ebfdae0 100644
--- a/zilencer/management/commands/populate_db.py
+++ b/zilencer/management/commands/populate_db.py
@@ -366,7 +366,7 @@ class Command(BaseCommand):
                 for profile in profiles:
                     email = profile.delivery_email
                     if email not in subscriptions_map:
-                        raise Exception('Subscriptions not listed for user %s' % (email,))
+                        raise Exception(f'Subscriptions not listed for user {email}')
 
                     for stream_name in subscriptions_map[email]:
                         stream = Stream.objects.get(name=stream_name)
diff --git a/zilencer/management/commands/print_initial_password.py b/zilencer/management/commands/print_initial_password.py
index 3c728a5f33..aab724290a 100644
--- a/zilencer/management/commands/print_initial_password.py
+++ b/zilencer/management/commands/print_initial_password.py
@@ -21,7 +21,7 @@ class Command(ZulipBaseCommand):
         print(self.fmt % ('email', 'password', 'API key'))
         for email in options['emails']:
             if '@' not in email:
-                print('ERROR: %s does not look like an email address' % (email,))
+                print(f'ERROR: {email} does not look like an email address')
                 continue
             user = self.get_user(email, realm)
             print(self.fmt % (email, initial_password(email), get_api_key(user)))
diff --git a/zilencer/management/commands/sync_api_key.py b/zilencer/management/commands/sync_api_key.py
index 6cfade03ac..619781aaa3 100644
--- a/zilencer/management/commands/sync_api_key.py
+++ b/zilencer/management/commands/sync_api_key.py
@@ -26,4 +26,4 @@ class Command(BaseCommand):
             user_profile.api_key = api_key
             user_profile.save(update_fields=["api_key"])
         except UserProfile.DoesNotExist:
-            print("User %s does not exist; not syncing API key" % (email,))
+            print(f"User {email} does not exist; not syncing API key")
diff --git a/zilencer/models.py b/zilencer/models.py
index b913b64759..91f93606cb 100644
--- a/zilencer/models.py
+++ b/zilencer/models.py
@@ -21,7 +21,7 @@ class RemoteZulipServer(models.Model):
     last_updated: datetime.datetime = models.DateTimeField('last updated', auto_now=True)
 
     def __str__(self) -> str:
-        return "<RemoteZulipServer %s %s>" % (self.hostname, self.uuid[0:12])
+        return f"<RemoteZulipServer {self.hostname} {self.uuid[0:12]}>"
 
     def format_requestor_for_logs(self) -> str:
         return "zulip-server:" + self.uuid
@@ -36,7 +36,7 @@ class RemotePushDeviceToken(AbstractPushDeviceToken):
         unique_together = ("server", "user_id", "kind", "token")
 
     def __str__(self) -> str:
-        return "<RemotePushDeviceToken %s %s>" % (self.server, self.user_id)
+        return f"<RemotePushDeviceToken {self.server} {self.user_id}>"
 
 class RemoteRealmAuditLog(AbstractRealmAuditLog):
     """Synced audit data from a remote Zulip server, used primarily for
@@ -48,8 +48,7 @@ class RemoteRealmAuditLog(AbstractRealmAuditLog):
     remote_id: int = models.IntegerField(db_index=True)
 
     def __str__(self) -> str:
-        return "<RemoteRealmAuditLog: %s %s %s %s>" % (
-            self.server, self.event_type, self.event_time, self.id)
+        return f"<RemoteRealmAuditLog: {self.server} {self.event_type} {self.event_time} {self.id}>"
 
 class RemoteInstallationCount(BaseCount):
     server: RemoteZulipServer = models.ForeignKey(RemoteZulipServer, on_delete=models.CASCADE)
@@ -63,7 +62,7 @@ class RemoteInstallationCount(BaseCount):
         ]
 
     def __str__(self) -> str:
-        return "<InstallationCount: %s %s %s>" % (self.property, self.subgroup, self.value)
+        return f"<InstallationCount: {self.property} {self.subgroup} {self.value}>"
 
 # We can't subclass RealmCount because we only have a realm_id here, not a foreign key.
 class RemoteRealmCount(BaseCount):
@@ -80,4 +79,4 @@ class RemoteRealmCount(BaseCount):
         ]
 
     def __str__(self) -> str:
-        return "%s %s %s %s %s" % (self.server, self.realm_id, self.property, self.subgroup, self.value)
+        return f"{self.server} {self.realm_id} {self.property} {self.subgroup} {self.value}"
diff --git a/zproject/backends.py b/zproject/backends.py
index e23d84bb60..04cfcfdd83 100644
--- a/zproject/backends.py
+++ b/zproject/backends.py
@@ -143,7 +143,7 @@ def any_social_backend_enabled(realm: Optional[Realm]=None) -> bool:
     return auth_enabled_helper(social_backend_names, realm)
 
 def redirect_to_config_error(error_type: str) -> HttpResponseRedirect:
-    return HttpResponseRedirect("/config-error/%s" % (error_type,))
+    return HttpResponseRedirect(f"/config-error/{error_type}")
 
 def require_email_format_usernames(realm: Optional[Realm]=None) -> bool:
     if ldap_auth_enabled(realm):
@@ -445,8 +445,7 @@ class ZulipLDAPAuthBackendBase(ZulipAuthMixin, LDAPBackend):
         if settings.LDAP_APPEND_DOMAIN:
             if is_valid_email(username):
                 if not username.endswith("@" + settings.LDAP_APPEND_DOMAIN):
-                    raise ZulipLDAPExceptionOutsideDomain("Email %s does not match LDAP domain %s." % (
-                        username, settings.LDAP_APPEND_DOMAIN))
+                    raise ZulipLDAPExceptionOutsideDomain(f"Email {username} does not match LDAP domain {settings.LDAP_APPEND_DOMAIN}.")
                 result = email_to_username(username)
         else:
             # We can use find_ldap_users_by_email
@@ -485,8 +484,7 @@ class ZulipLDAPAuthBackendBase(ZulipAuthMixin, LDAPBackend):
         if settings.LDAP_EMAIL_ATTR is not None:
             # Get email from ldap attributes.
             if settings.LDAP_EMAIL_ATTR not in ldap_user.attrs:
-                raise ZulipLDAPException("LDAP user doesn't have the needed %s attribute" % (
-                    settings.LDAP_EMAIL_ATTR,))
+                raise ZulipLDAPException(f"LDAP user doesn't have the needed {settings.LDAP_EMAIL_ATTR} attribute")
             else:
                 return ldap_user.attrs[settings.LDAP_EMAIL_ATTR][0]
 
@@ -611,12 +609,12 @@ class ZulipLDAPAuthBackendBase(ZulipAuthMixin, LDAPBackend):
             try:
                 field = fields_by_var_name[var_name]
             except KeyError:
-                raise ZulipLDAPException('Custom profile field with name %s not found.' % (var_name,))
+                raise ZulipLDAPException(f'Custom profile field with name {var_name} not found.')
             if existing_values.get(var_name) == value:
                 continue
             result = validate_user_custom_profile_field(user_profile.realm.id, field, value)
             if result is not None:
-                raise ZulipLDAPException('Invalid data for %s field: %s' % (var_name, result))
+                raise ZulipLDAPException(f'Invalid data for {var_name} field: {result}')
             profile_data.append({
                 'id': field.id,
                 'value': value,
@@ -876,9 +874,9 @@ def query_ldap(email: str) -> List[str]:
             if django_field == "avatar":
                 if isinstance(value, bytes):
                     value = "(An avatar image file)"
-            values.append("%s: %s" % (django_field, value))
+            values.append(f"{django_field}: {value}")
         if settings.LDAP_EMAIL_ATTR is not None:
-            values.append("%s: %s" % ('email', ldap_attrs[settings.LDAP_EMAIL_ATTR][0]))
+            values.append("{}: {}".format('email', ldap_attrs[settings.LDAP_EMAIL_ATTR][0]))
     else:
         values.append("LDAP backend not configured on this server.")
     return values
@@ -1680,7 +1678,7 @@ class SAMLAuthBackend(SocialAuthMixin, SAMLAuth):
             if idps_without_limit_to_subdomains:
                 self.logger.error("SAML_REQUIRE_LIMIT_TO_SUBDOMAINS is enabled and the following " +
                                   "IdPs don't have limit_to_subdomains specified and will be ignored: " +
-                                  "%s" % (idps_without_limit_to_subdomains,))
+                                  f"{idps_without_limit_to_subdomains}")
                 for idp_name in idps_without_limit_to_subdomains:
                     del settings.SOCIAL_AUTH_SAML_ENABLED_IDPS[idp_name]
         super().__init__(*args, **kwargs)
@@ -1874,7 +1872,7 @@ class SAMLAuthBackend(SocialAuthMixin, SAMLAuth):
     def validate_idp_for_subdomain(cls, idp_name: str, subdomain: str) -> bool:
         idp_dict = settings.SOCIAL_AUTH_SAML_ENABLED_IDPS.get(idp_name)
         if idp_dict is None:
-            raise AssertionError("IdP: %s not found" % (idp_name,))
+            raise AssertionError(f"IdP: {idp_name} not found")
         if 'limit_to_subdomains' in idp_dict and subdomain not in idp_dict['limit_to_subdomains']:
             return False
 
diff --git a/zproject/urls.py b/zproject/urls.py
index 1081f51a12..48686c3b94 100644
--- a/zproject/urls.py
+++ b/zproject/urls.py
@@ -696,7 +696,7 @@ urls += [
 for app_name in settings.EXTRA_INSTALLED_APPS:
     app_dir = os.path.join(settings.DEPLOY_ROOT, app_name)
     if os.path.exists(os.path.join(app_dir, 'urls.py')):
-        urls += [url(r'^', include('%s.urls' % (app_name,)))]
+        urls += [url(r'^', include(f'{app_name}.urls'))]
         i18n_urls += import_string(f"{app_name}.urls.i18n_urlpatterns")
 
 # Tornado views