      def to_representation(self, value):
          if self.format is None:
              return value
  
          if self.format.lower() == ISO_8601:
              value = value.isoformat()
              if value.endswith('+00:00'):
                  value = value[:-6] + 'Z'
              return value
          return value.strftime(self.format)
 
</s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format) </s> remove         if self.format is None:
 </s> add         output_format = getattr(self, 'format', api_settings.DATETIME_FORMAT)

        if output_format is None: </s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601: </s> remove         return value.strftime(self.format)
 </s> add         return value.strftime(output_format) </s> remove         if self.format.lower() == ISO_8601:
 </s> add         if output_format.lower() == ISO_8601:
        if output_format.lower() == ISO_8601:
      def to_representation(self, value):
          if self.format is None:
              return value
  
        if output_format.lower() == ISO_8601:
              value = value.isoformat()
              if value.endswith('+00:00'):
                  value = value[:-6] + 'Z'
              return value
          return value.strftime(self.format)
keep keep keep keep replace keep keep keep keep keep
 Access settings lazily, not at module import 
rest_framework/fields.py