code: def update_payload(self, fields=None):
          
          values = self.get_values()
          if fields is not None:
              values = {field: values[field] for field in fields}
          return _payload(self.get_fields(), values)
docs: Create a payload of values that can be sent to the server.
  
          By default, this method behaves just like :func:`_payload`. However,
          one can also specify a certain set of fields that should be returned.
          For more information, see :meth:`update`.