<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update_payload(self, fields=None):
 """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`. """

        values = self.get_values()
        if fields is not None:
            values = {field: values[field] for field in fields}
        return _payload(self.get_fields(), values)