code: def _check_once(self):
          
          address = self._server_description.address
          if self._publish:
              self._listeners.publish_server_heartbeat_started(address)
          with self._pool.get_socket({}) as sock_info:
              response, round_trip_time = self._check_with_socket(sock_info)
              self._avg_round_trip_time.add_sample(round_trip_time)
              sd = ServerDescription(
                  address=address,
                  ismaster=response,
                  round_trip_time=self._avg_round_trip_time.get())
              if self._publish:
                  self._listeners.publish_server_heartbeat_succeeded(
                      address, round_trip_time, response)
  
              return sd
docs: A single attempt to call ismaster.
  
          Returns a ServerDescription, or raises an exception.