def uid(self):
        """str: A unique identifier.

        Looks like: ``'RINCON_000XXXXXXXXXX1400'``
        """
        # Since this does not change over time (?) check whether we already
        # know the answer. If so, there is no need to go further
        if self._uid is not None:
            return self._uid
        # if not, we have to get it from the zone topology, which
        # is probably quicker than any alternative, since the zgt is probably
        # cached. This will set self._uid for us for next time, so we won't
        # have to do this again
        self._parse_zone_group_state()
        return self._uid
str: A unique identifier.

        Looks like: ``'RINCON_000XXXXXXXXXX1400'``