diff --git a/scripts/setup/generate_secrets.py b/scripts/setup/generate_secrets.py
index 1c49548ead..41b04a88bc 100755
--- a/scripts/setup/generate_secrets.py
+++ b/scripts/setup/generate_secrets.py
@@ -47,7 +47,7 @@ def generate_django_secretkey():
     return get_random_string(50, chars)
 
 def get_old_conf(output_filename):
-    # type: (Text) -> Dict[str, Text]
+    # type: (str) -> Dict[str, Text]
     if not os.path.exists(output_filename):
         return {}
 
@@ -55,7 +55,7 @@ def get_old_conf(output_filename):
     secrets_file.read(output_filename)
 
     def get_secret(key):
-        # type: (Text) -> Optional[Text]
+        # type: (str) -> Optional[Text]
         if secrets_file.has_option('secrets', key):
             return secrets_file.get('secrets', key)
         return None
diff --git a/tools/documentation_crawler/documentation_crawler/spiders/check_documentation.py b/tools/documentation_crawler/documentation_crawler/spiders/check_documentation.py
index 0a294c7d32..ca55c5efe0 100755
--- a/tools/documentation_crawler/documentation_crawler/spiders/check_documentation.py
+++ b/tools/documentation_crawler/documentation_crawler/spiders/check_documentation.py
@@ -23,5 +23,5 @@ def get_start_url():
 class DocumentationSpider(BaseDocumentationSpider):
     name = "documentation_crawler"
     deny_domains = ['localhost:9991']
-    deny = '\_sources\/.*\.txt'
+    deny = ['\_sources\/.*\.txt']
     start_urls = get_start_url()
diff --git a/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py b/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
index d54b556b73..c21122828e 100644
--- a/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
+++ b/tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py
@@ -18,7 +18,7 @@ class BaseDocumentationSpider(scrapy.Spider):
     # Exclude domain address.
     deny_domains = []  # type: List[str]
     start_urls = []  # type: List[str]
-    deny = ()  # type: Tuple
+    deny = []  # type: List[str]
     file_extensions = ['.' + ext for ext in IGNORED_EXTENSIONS]  # type: List[str]
     tags = ('a', 'area', 'img')
     attrs = ('href', 'src')
diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index bc2e9c8206..2c072e01ed 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -82,7 +82,7 @@ from zerver.lib.session_user import get_session_user
 from zerver.lib.upload import attachment_url_re, attachment_url_to_path_id, \
     claim_attachment, delete_message_image
 from zerver.lib.str_utils import NonBinaryStr, force_str
-from zerver.tornado.event_queue import request_event_queue, get_user_events, send_event
+from zerver.tornado.event_queue import request_event_queue, send_event
 
 import DNS
 import ujson
diff --git a/zerver/lib/events.py b/zerver/lib/events.py
index 5204ae7355..d70de6d96c 100644
--- a/zerver/lib/events.py
+++ b/zerver/lib/events.py
@@ -419,7 +419,7 @@ def do_events_register(user_profile, user_client, apply_markdown=True,
     # Apply events that came in while we were fetching initial data
     events = get_user_events(user_profile, queue_id, -1)
     apply_events(ret, events, user_profile, include_subscribers=include_subscribers)
-    if events:
+    if len(events) > 0:
         ret['last_event_id'] = events[-1]['id']
     else:
         ret['last_event_id'] = -1
diff --git a/zerver/tornado/event_queue.py b/zerver/tornado/event_queue.py
index 09143c9da3..3f8d1f79dd 100644
--- a/zerver/tornado/event_queue.py
+++ b/zerver/tornado/event_queue.py
@@ -606,7 +606,7 @@ def get_user_events(user_profile, queue_id, last_event_id):
         resp.raise_for_status()
 
         return extract_json_response(resp)['events']
-
+    return []
 
 # Send email notifications to idle users
 # after they are idle for 1 hour
diff --git a/zerver/webhooks/github/view.py b/zerver/webhooks/github/view.py
index 8f9bd77940..b07eb31274 100644
--- a/zerver/webhooks/github/view.py
+++ b/zerver/webhooks/github/view.py
@@ -92,10 +92,11 @@ def get_pull_request_or_issue_action(payload):
     return 'synchronized' if payload['action'] == 'synchronize' else payload['action']
 
 def get_pull_request_or_issue_assignee(object_payload):
-    # type: (Mapping[Text, Any]) -> Text
+    # type: (Mapping[Text, Any]) -> Optional[Text]
     assignee_dict = object_payload.get('assignee')
     if assignee_dict:
         return assignee_dict.get('login')
+    return None
 
 def get_pull_request_or_issue_subject(repository, payload_object, type):
     # type: (Mapping[Text, Any], Mapping[Text, Any], Text) -> Text
diff --git a/zerver/webhooks/gitlab/view.py b/zerver/webhooks/gitlab/view.py
index 43755701f5..cb8db602d5 100644
--- a/zerver/webhooks/gitlab/view.py
+++ b/zerver/webhooks/gitlab/view.py
@@ -115,10 +115,11 @@ def get_merge_request_open_or_updated_body(payload, action):
     )
 
 def get_objects_assignee(payload):
-    # type: (Dict[str, Any]) -> Text
+    # type: (Dict[str, Any]) -> Optional[Text]
     assignee_object = payload.get('assignee')
     if assignee_object:
         return assignee_object.get('name')
+    return None
 
 def get_commented_commit_event_body(payload):
     # type: (Dict[str, Any]) -> Text