diff --git a/configcatclient/configcatoptions.py b/configcatclient/configcatoptions.py
index 6cd74eb..896c863 100644
--- a/configcatclient/configcatoptions.py
+++ b/configcatclient/configcatoptions.py
@@ -75,7 +75,7 @@ class ConfigCatOptions(object):
 
     def __init__(self,
                  base_url=None,
-                 polling_mode=PollingMode.auto_poll(),
+                 polling_mode: PollingMode = PollingMode.auto_poll(),
                  config_cache=None,
                  proxies=None,
                  proxy_auth=None,
diff --git a/configcatclient/configfetcher.py b/configcatclient/configfetcher.py
index fc6c248..59618bc 100644
--- a/configcatclient/configfetcher.py
+++ b/configcatclient/configfetcher.py
@@ -19,14 +19,14 @@
 
 
 class RedirectMode(IntEnum):
-    NoRedirect = 0,
-    ShouldRedirect = 1,
+    NoRedirect = 0
+    ShouldRedirect = 1
     ForceRedirect = 2
 
 
 class Status(IntEnum):
-    Fetched = 0,
-    NotModified = 1,
+    Fetched = 0
+    NotModified = 1
     Failure = 2
 
 
diff --git a/configcatclient/overridedatasource.py b/configcatclient/overridedatasource.py
index 0237f57..bdb9213 100644
--- a/configcatclient/overridedatasource.py
+++ b/configcatclient/overridedatasource.py
@@ -5,12 +5,12 @@
 class OverrideBehaviour(IntEnum):
     # When evaluating values, the SDK will not use feature flags & settings from the ConfigCat CDN, but it will use
     # all feature flags & settings that are loaded from local-override sources.
-    LocalOnly = 0,
+    LocalOnly = 0
 
     # When evaluating values, the SDK will use all feature flags & settings that are downloaded from the ConfigCat CDN,
     # plus all feature flags & settings that are loaded from local-override sources. If a feature flag or a setting is
     # defined both in the fetched and the local-override source then the local-override version will take precedence.
-    LocalOverRemote = 1,
+    LocalOverRemote = 1
 
     # When evaluating values, the SDK will use all feature flags & settings that are downloaded from the ConfigCat CDN,
     # plus all feature flags & settings that are loaded from local-override sources. If a feature flag or a setting is
diff --git a/configcatclient/pollingmode.py b/configcatclient/pollingmode.py
index e99e7e2..f41ca01 100644
--- a/configcatclient/pollingmode.py
+++ b/configcatclient/pollingmode.py
@@ -8,7 +8,7 @@ class PollingMode(object):
     __metaclass__ = ABCMeta
 
     @abstractmethod
-    def identifier(self):
+    def identifier(self) -> str:
         """
         :return: the identifier of polling mode. Used for analytical purposes in HTTP User-Agent headers.
         """