diff --git a/provision.py b/provision.py
index b5ee137586..414eb7a66e 100644
--- a/provision.py
+++ b/provision.py
@@ -110,14 +110,14 @@ REPO_STOPWORDS_PATH = os.path.join(
 
 LOUD = dict(_out=sys.stdout, _err=sys.stderr)
 
-def setup_virtualenv(target_venv_path, requirements_file, virtualenv_args=[]):
+def setup_virtualenv(target_venv_path, requirements_file, virtualenv_args=None):
     # Check if a cached version already exists
     output = subprocess.check_output(['sha1sum', requirements_file])
     sha1sum = output.split()[0]
     cached_venv_path = os.path.join(VENV_CACHE_PATH, sha1sum, os.path.basename(target_venv_path))
     success_stamp = os.path.join(cached_venv_path, "success-stamp")
     if not os.path.exists(success_stamp):
-        do_setup_virtualenv(cached_venv_path, requirements_file, virtualenv_args)
+        do_setup_virtualenv(cached_venv_path, requirements_file, virtualenv_args or [])
         run(["touch", success_stamp])
 
     print("Using cached Python venv from %s" % (cached_venv_path,))
diff --git a/zerver/lib/bugdown/codehilite.py b/zerver/lib/bugdown/codehilite.py
index a771631ad5..2416e79ed0 100644
--- a/zerver/lib/bugdown/codehilite.py
+++ b/zerver/lib/bugdown/codehilite.py
@@ -220,6 +220,6 @@ class CodeHiliteExtension(markdown.Extension):
         md.registerExtension(self)
 
 
-def makeExtension(configs={}):
-  return CodeHiliteExtension(configs=configs)
+def makeExtension(configs=None):
+  return CodeHiliteExtension(configs=configs or [])
 
diff --git a/zerver/tests/test_auth_backends.py b/zerver/tests/test_auth_backends.py
index b1a8ee4898..8102775b60 100644
--- a/zerver/tests/test_auth_backends.py
+++ b/zerver/tests/test_auth_backends.py
@@ -22,9 +22,13 @@ from zproject.backends import ZulipDummyBackend, EmailAuthBackend, \
 import ujson
 
 class AuthBackendTest(TestCase):
-    def verify_backend(self, backend, good_args={},
-                       good_kwargs={}, bad_kwargs=None,
+    def verify_backend(self, backend, good_args=None,
+                       good_kwargs=None, bad_kwargs=None,
                        email_to_username=None):
+        if good_args is None:
+            good_args = []
+        if good_kwargs is None:
+            good_kwargs = {}
         email = "hamlet@zulip.com"
         user_profile = get_user_profile_by_email(email)