How to use the placebo.serializer.Format.write_mode function in placebo

http_response=200):
        """
        Store a response to the data directory.  The ``operation``
        should be the name of the operation in the service API (e.g.
        DescribeInstances), the ``response_data`` should a value you want
        to return from a placebo call and the ``http_response`` should be
        the HTTP status code returned from the service.  You can add
        multiple responses for a given operation and they will be
        returned in order.
        """
        LOG.debug('save_response: %s.%s', service, operation)
        filepath = self.get_new_file_path(service, operation)
        LOG.debug('save_response: path=%s', filepath)
        data = {'status_code': http_response,
                'data': response_data}
        with open(filepath, Format.write_mode(self.record_format)) as fp:
            self._serializer(data, fp)