diff --git a/contrib/buildbot/shieldio.py b/contrib/buildbot/shieldio.py
--- a/contrib/buildbot/shieldio.py
+++ b/contrib/buildbot/shieldio.py
@@ -31,7 +31,8 @@
             scheme,
             netloc,
             path,
-            unquote(urlencode({**self.query, **kwargs}, doseq=True, quote_via=quote)),
+            unquote(urlencode({**self.query, **kwargs},
+                              doseq=True, quote_via=quote)),
             ''
         ))
 
diff --git a/contrib/buildbot/test/mocks/phabricator.py b/contrib/buildbot/test/mocks/phabricator.py
--- a/contrib/buildbot/test/mocks/phabricator.py
+++ b/contrib/buildbot/test/mocks/phabricator.py
@@ -10,7 +10,7 @@
 
 
 class Result:
-    def __init__(self, data=[]):
+    def __init__(self, data: list):
         self.data = data
 
 
diff --git a/contrib/buildbot/test/mocks/slackbot.py b/contrib/buildbot/test/mocks/slackbot.py
--- a/contrib/buildbot/test/mocks/slackbot.py
+++ b/contrib/buildbot/test/mocks/slackbot.py
@@ -47,9 +47,8 @@
     }
 
 
-def users_list(total=1, initialUsers=[]):
-    users = []
-    users.extend(initialUsers)
+def users_list(total=1, initialUsers=None):
+    users = initialUsers if initialUsers is not None else []
     for i in range(len(users), total):
         users.append(user('U{}'.format(DEFAULT_USER_NUM + i)))
     return {
diff --git a/contrib/teamcity/build-configurations.py b/contrib/teamcity/build-configurations.py
--- a/contrib/teamcity/build-configurations.py
+++ b/contrib/teamcity/build-configurations.py
@@ -369,7 +369,8 @@
                 )
                 continue
 
-    def run_process(self, binary, args=[]):
+    def run_process(self, binary, args=None):
+        args = args if args is not None else []
         return asyncio.create_subprocess_exec(
             *([binary] + args),
             # Buffer limit is 64KB by default, but we need a larger buffer:
@@ -386,7 +387,8 @@
             },
         )
 
-    async def run_build(self, binary, args=[]):
+    async def run_build(self, binary, args=None):
+        args = args if args is not None else []
         proc = await self.run_process(binary, args)
 
         await asyncio.wait([
@@ -395,7 +397,8 @@
 
         return await proc.wait()
 
-    async def wait_for_build(self, timeout, args=[]):
+    async def wait_for_build(self, timeout, args=None):
+        args = args if args is not None else []
         message = "Build {} completed successfully".format(
             self.configuration.name
         )
@@ -434,7 +437,8 @@
 
             return (return_code, message)
 
-    def run(self, args=[]):
+    def run(self, args=None):
+        args = args if args is not None else []
         if self.artifact_dir.is_dir():
             shutil.rmtree(self.artifact_dir)
         self.artifact_dir.mkdir(exist_ok=True)
@@ -478,7 +482,9 @@
         )
         self.teamcity_messages.publishArtifacts(artifact_path_pattern)
 
-    def run(self, args=[]):
+    def run(self, args=None):
+        args = args if args is not None else []
+
         # Let the user know what build is being run.
         # This makes it easier to retrieve the info from the logs.
         self.teamcity_messages.customMessage(