docs: def get_zone_variable(self, zone_id, variable):
          """ Retrieve the current value of a zone variable.  If the variable is
          not found in the local cache then the value is requested from the
          controller.  """
  
          try:
              return self._retrieve_cached_zone_variable(zone_id, variable)
          except UncachedVariable:
              return (yield from self._send_cmd("GET %s.%s" % (
                  zone_id.device_str(), variable)))