diff --git a/api/bin/zulip-send b/api/bin/zulip-send
index 5d3adbd288..d85b47cea1 100755
--- a/api/bin/zulip-send
+++ b/api/bin/zulip-send
@@ -122,7 +122,7 @@ def main(argv=None):
 
     if not do_send_message(client, message_data):
         return 1
-
+    return 0
 
 if __name__ == '__main__':
     sys.exit(main())
diff --git a/api/integrations/hg/zulip-changegroup.py b/api/integrations/hg/zulip-changegroup.py
index c0bb9cac49..21dc923cf4 100755
--- a/api/integrations/hg/zulip-changegroup.py
+++ b/api/integrations/hg/zulip-changegroup.py
@@ -87,7 +87,7 @@ def format_commit_lines(web_url, repo, base, tip):
     return "\n".join(summary for summary in commit_summaries)
 
 def send_zulip(email, api_key, site, stream, subject, content):
-    # type: (str, str, str, str, str, Text) -> str
+    # type: (str, str, str, str, str, Text) -> None
     """
     Send a message to Zulip using the provided credentials, which should be for
     a bot in most cases.
diff --git a/scripts/lib/zulip_tools.py b/scripts/lib/zulip_tools.py
index e10a26f225..1d9bcbb726 100755
--- a/scripts/lib/zulip_tools.py
+++ b/scripts/lib/zulip_tools.py
@@ -86,7 +86,7 @@ def release_deployment_lock():
     shutil.rmtree(LOCK_DIR)
 
 def run(args, **kwargs):
-    # type: (Sequence[str], **Any) -> int
+    # type: (Sequence[str], **Any) -> None
     # Output what we're doing in the `set -x` style
     print("+ %s" % (" ".join(args)))
 
diff --git a/tools/check-urls b/tools/check-urls
index 892f3add77..cae048bfee 100755
--- a/tools/check-urls
+++ b/tools/check-urls
@@ -7,7 +7,7 @@ import sys
 
 
 def check_urls():
-    # type: () -> bool
+    # type: () -> None
     url_files = ['zproject/urls.py',
                  'zproject/dev_urls.py',
                  'zproject/legacy_urls.py',
diff --git a/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py b/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
index 452b9f717a..d54b556b73 100644
--- a/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
+++ b/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
@@ -97,3 +97,4 @@ class BaseDocumentationSpider(scrapy.Spider):
             self._set_error_state()
         else:
             raise Exception(failure.value)
+        return None
diff --git a/tools/lib/find_add_class.py b/tools/lib/find_add_class.py
index ae3f037ec2..0755254d1c 100644
--- a/tools/lib/find_add_class.py
+++ b/tools/lib/find_add_class.py
@@ -57,6 +57,7 @@ def generic(html_class):
     for kw in GENERIC_KEYWORDS:
         if kw in html_class:
             return True
+    return False
 
 def display(fns):
     # type: (List[str]) -> None
diff --git a/tools/lib/html_branches.py b/tools/lib/html_branches.py
index 528a294dc9..9a2b927dad 100644
--- a/tools/lib/html_branches.py
+++ b/tools/lib/html_branches.py
@@ -144,13 +144,11 @@ def split_for_id_and_class(element):
 
 def html_branches(text, fn=None):
     # type: (str, str) -> List[HtmlTreeBranch]
-
     tree = html_tag_tree(text)
     branches = []  # type: List[HtmlTreeBranch]
 
     def walk(node, tag_info_list=None):
-        # type: (Node, Optional[List[TagInfo]]) -> Node
-
+        # type: (Node, Optional[List[TagInfo]]) -> None
         info = get_tag_info(node.token)
         if tag_info_list is None:
             tag_info_list = [info]
@@ -195,7 +193,6 @@ def html_tag_tree(text):
 
 def build_id_dict(templates):
     # type: (List[str]) -> (Dict[str,List[str]])
-
     template_id_dict = defaultdict(list) # type: (Dict[str,List[str]])
 
     for fn in templates:
diff --git a/tools/run-dev.py b/tools/run-dev.py
index 86e1492cbf..c0ecbc1e03 100755
--- a/tools/run-dev.py
+++ b/tools/run-dev.py
@@ -241,6 +241,7 @@ class BaseWebsocketHandler(WebSocketHandler):
             # close websocket proxy connection if no connection with target websocket server
             return self.close()
         self.client.write_message(message, binary)
+        return None
 
     def check_origin(self, origin):
         # type: (str) -> bool
@@ -262,6 +263,7 @@ class CombineHandler(BaseWebsocketHandler):
         # type: (*Any, **Any) -> Optional[Callable]
         if self.request.headers.get("Upgrade", "").lower() == 'websocket':
             return super(CombineHandler, self).get(*args, **kwargs)
+        return None
 
     def head(self):
         # type: () -> None
diff --git a/zerver/lib/bugdown/__init__.py b/zerver/lib/bugdown/__init__.py
index ac112865d9..d7b6929b0f 100644
--- a/zerver/lib/bugdown/__init__.py
+++ b/zerver/lib/bugdown/__init__.py
@@ -402,6 +402,7 @@ class InlineInterestingLinkProcessor(markdown.treeprocessors.Treeprocessor):
 
         if yt_id is not None:
             return "https://i.ytimg.com/vi/%s/default.jpg" % (yt_id,)
+        return None
 
     def twitter_text(self, text, urls, user_mentions, media):
         # type: (Text, List[Dict[Text, Text]], List[Dict[Text, Any]], List[Dict[Text, Any]]) -> Element
@@ -972,7 +973,7 @@ class UserMentionPattern(markdown.inlinepatterns.Pattern):
             el.set('data-user-id', user_id)
             el.text = "@%s" % (name,)
             return el
-
+        return None
 
 class StreamPattern(VerbosePattern):
     def find_stream_by_name(self, name):
@@ -1001,7 +1002,7 @@ class StreamPattern(VerbosePattern):
                 stream_name=urllib.parse.quote(force_str(name))))
             el.text = u'#{stream_name}'.format(stream_name=name)
             return el
-
+        return None
 
 class AlertWordsNotificationProcessor(markdown.preprocessors.Preprocessor):
     def run(self, lines):
diff --git a/zerver/lib/i18n.py b/zerver/lib/i18n.py
index fac8a7683f..fa3d5f92c2 100644
--- a/zerver/lib/i18n.py
+++ b/zerver/lib/i18n.py
@@ -72,6 +72,7 @@ def get_language_name(code):
     for lang in get_language_list():
         if lang['code'] == code:
             return lang['name']
+    return None
 
 def get_available_language_codes():
     # type: () -> List[Text]
diff --git a/zerver/lib/test_helpers.py b/zerver/lib/test_helpers.py
index b31e3fb4df..7c8cf4a6f7 100644
--- a/zerver/lib/test_helpers.py
+++ b/zerver/lib/test_helpers.py
@@ -194,20 +194,22 @@ def make_client(name):
     return client
 
 def find_key_by_email(address):
-    # type: (Text) -> Text
+    # type: (Text) -> Optional[Text]
     from django.core.mail import outbox
     key_regex = re.compile("accounts/do_confirm/([a-f0-9]{40})>")
     for message in reversed(outbox):
         if address in message.to:
             return key_regex.search(message.body).groups()[0]
+    return None
 
 def find_pattern_in_email(address, pattern):
-    # type: (Text, Text) -> Text
+    # type: (Text, Text) -> Optional[Text]
     from django.core.mail import outbox
     key_regex = re.compile(pattern)
     for message in reversed(outbox):
         if address in message.to:
             return key_regex.search(message.body).group(0)
+    return None
 
 def message_ids(result):
     # type: (Dict[str, Any]) -> Set[int]
diff --git a/zerver/lib/utils.py b/zerver/lib/utils.py
index 58c1bb44fa..31a5d957a8 100644
--- a/zerver/lib/utils.py
+++ b/zerver/lib/utils.py
@@ -41,7 +41,7 @@ class StatsDWrapper(object):
     # as our statsd server supports them but supporting
     # pystatsd is not released yet
     def _our_gauge(self, stat, value, rate=1, delta=False):
-            # type: (str, float, float, bool) -> str
+            # type: (str, float, float, bool) -> None
             """Set a gauge value."""
             from django_statsd.clients import statsd
             if delta:
diff --git a/zerver/models.py b/zerver/models.py
index c283430638..3e221f20c3 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -1354,6 +1354,8 @@ class UserPresence(models.Model):
             return 'active'
         elif status == UserPresence.IDLE:
             return 'idle'
+        else:
+            raise ValueError('Unknown status: %s' % (status,))
 
     @staticmethod
     def get_status_dict_by_user(user_profile):
diff --git a/zerver/tests/test_decorators.py b/zerver/tests/test_decorators.py
index 7de8d4cacc..c37f1f568d 100644
--- a/zerver/tests/test_decorators.py
+++ b/zerver/tests/test_decorators.py
@@ -470,6 +470,7 @@ class ValidatorTestCase(TestCase):
             ])('_', val)
             if error:
                 return 'This is not a valid person'
+            return None
 
         person = {'name': 'King Lear', 'age': 42}
         self.assertEqual(check_person(person), None)
diff --git a/zproject/backends.py b/zproject/backends.py
index 345f061ed6..5cd3e85d04 100644
--- a/zproject/backends.py
+++ b/zproject/backends.py
@@ -278,6 +278,7 @@ class GoogleMobileOauth2Backend(ZulipAuthMixin):
             return user_profile
         else:
             return_data["valid_attestation"] = False
+            return None
 
 class ZulipRemoteUserBackend(RemoteUserBackend):
     create_unknown_user = False