bool: Is this zone a bridge?

def is_bridge(self):
        """bool: Is this zone a bridge?"""
        # 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._is_bridge is not None:
            return self._is_bridge
        # if not, we have to get it from the zone topology. This will set
        # self._is_bridge for us for next time, so we won't have to do this
        # again
        self._parse_zone_group_state()
        return self._is_bridge