def format(self, value):
        try:
            if self.dt_format is 'rfc822':
                return _rfc822(value)
            elif self.dt_format == 'iso8601':
                return _iso8601(value)
            else:
                raise MarshallingException(
                    'Unsupported date format %s' % self.dt_format
                )
        except AttributeError as ae:
            raise MarshallingException(ae)
dataset/ETHPy150Open flask-restful/flask-restful/flask_restful/fields.py/DateTime.format/operator `==` -> `is`