<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def port(self, value=None):
 """ Return or set the port :param string value: the new port to use :returns: string or new :class:`URL` instance """

        if value is not None:
            return URL._mutate(self, port=value)
        return self._tuple.port