class NsdmSubscriptionLinkSerializer(serializers.Serializer):
     self = LinkSerializer(
         help_text="Links to resources related to this resource.",
-        required=True)
+        required=True
+    )
 
 
 class NsdmSubscriptionSerializer(serializers.Serializer):
         help_text="Identifier of this subscription resource.",
         max_length=255,
         required=True,
-        allow_null=False)
+        allow_null=False
+    )
     callbackUri = serializers.CharField(
         help_text="The URI of the endpoint to send the notification to.",
         max_length=255,
         required=True,
-        allow_null=False)
+        allow_null=False
+    )
     filter = NsdmNotificationsFilter(
         help_text="Filter settings for this subscription, to define the "
         "of all notifications this subscription relates to.",
-        required=False)
+        required=False
+    )
     _links = NsdmSubscriptionLinkSerializer(
         help_text="Links to resources related to this resource.",
-        required=True)
+        required=True
+    )
 
 
 class NsdmSubscriptionsSerializer(serializers.ListSerializer):
     subscription_id = serializers.UUIDField(
         help_text="Identifier of this subscription resource.",
         required=True,
-        allow_null=False)
+        allow_null=False
+    )
 
 
 class NsdmSubscriptionRequestSerializer(serializers.Serializer):
     callbackUri = serializers.CharField(
         help_text="The URI of the endpoint to send the notification to.",
         required=True,
-        allow_null=False)
+        allow_null=False
+    )
     filter = NsdmNotificationsFilter(
         help_text="Filter settings for the subscription,"
                   " to define the subset of all "
                   "notifications this subscription relates to.",
         required=False,
-        allow_null=True)
+        allow_null=True
+    )
     authentication = SubscriptionAuthenticationSerializer(
         help_text="Authentication parameters to configure"
                   " the use of Authorization when sending "
                   "notifications corresponding to this subscription.",
         required=False,
-        allow_null=True)
+        allow_null=True
+    )
 
 
 
 class ProblemDetailsSerializer(serializers.Serializer):
-    type = serializers.CharField(help_text="Type", required=False, allow_null=True)
-    title = serializers.CharField(help_text="Title", required=False, allow_null=True)
-    status = serializers.IntegerField(help_text="Status", required=True)
-    detail = serializers.CharField(help_text="Detail", required=True, allow_null=True)
-    instance = serializers.CharField(help_text="Instance", required=False, allow_null=True)
+    type = serializers.CharField(
+        help_text="Type",
+        required=False,
+        allow_null=True
+    )
+    title = serializers.CharField(
+        help_text="Title",
+        required=False,
+        allow_null=True
+    )
+    status = serializers.IntegerField(
+        help_text="Status",
+        required=True
+    )
+    detail = serializers.CharField(
+        help_text="Detail",
+        required=True,
+        allow_null=True
+    )
+    instance = serializers.CharField(
+        help_text="Instance",
+        required=False,
+        allow_null=True
+    )
     additional_details = serializers.ListField(
         help_text="Any number of additional attributes, as defined in a "
         "specification or by an implementation.",
         required=False,
-        allow_null=True)
+        allow_null=True
+    )
 
     class Meta:
         ref_name = 'SUBSCRIPTION_ProblemDetailsSerializer'
 
         "request of the OAuth 2.0 client credentials grant type.",
         required=False,
         max_length=255,
-        allow_null=False)
+        allow_null=False
+    )
     clientPassword = serializers.CharField(
         help_text="Client password to be used in the access token "
         "request of the OAuth 2.0 client credentials grant type.",
         required=False,
         max_length=255,
-        allow_null=False)
+        allow_null=False
+    )
     tokenEndpoint = serializers.CharField(
         help_text="The token endpoint from which the access token can "
         "be obtained.",
         required=False,
         max_length=255,
-        allow_null=False)
+        allow_null=False
+    )
 
 
 class BasicAuthSerializer(serializers.Serializer):
         help_text="Username to be used in HTTP Basic authentication.",
         max_length=255,
         required=False,
-        allow_null=False)
+        allow_null=False
+    )
     password = serializers.CharField(
         help_text="Password to be used in HTTP Basic authentication.",
         max_length=255,
         required=False,
-        allow_null=False)
+        allow_null=False
+    )
 
 
 class SubscriptionAuthenticationSerializer(serializers.Serializer):
         help_text="Defines the types of Authentication / Authorization "
         "which the API consumer is willing to accept when "
         "receiving a notification.",
-        required=True)
+        required=True
+    )
     paramsBasic = BasicAuthSerializer(
         help_text="Parameters for authentication/authorization using BASIC.",
         required=False,
-        allow_null=False)
+        allow_null=False
+    )
     paramsOauth2ClientCredentials = OAuthCredentialsSerializer(
         help_text="Parameters for authentication/authorization using "
         "OAUTH2_CLIENT_CREDENTIALS.",
         required=False,
-        allow_null=False)
+        allow_null=False
+    )