          used for `self.fields` when instantiating the serializer.
          """
          assert hasattr(self, 'Meta'), (
              'Class {serializer_class} missing "Meta" attribute'.format(
                  serializer_class=self.__class__.__name__
              )
          )
 
</s> remove     url_field_name = api_settings.URL_FIELD_NAME
 </s> add     url_field_name = None </s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format) </s> remove         self.user_settings = user_settings or {}
 </s> add         if user_settings:
            self._user_settings = user_settings </s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601: </s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601:
        if self.url_field_name is None:
            self.url_field_name = api_settings.URL_FIELD_NAME

          used for `self.fields` when instantiating the serializer.
          """
        if self.url_field_name is None:
            self.url_field_name = api_settings.URL_FIELD_NAME

          assert hasattr(self, 'Meta'), (
              'Class {serializer_class} missing "Meta" attribute'.format(
                  serializer_class=self.__class__.__name__
              )
          )
keep add keep keep keep keep keep
 Access settings lazily, not at module import 
rest_framework/serializers.py