def stopService(self):
        """
        Stop listening on the port if it is already listening, otherwise,
        cancel the attempt to listen.

        @return: a L{Deferred<twisted.internet.defer.Deferred>} which fires
            with L{None} when the port has stopped listening.
        """
        self._waitingForPort.cancel()
        def stopIt(port):
            if port is not None:
                return port.stopListening()
        d = self._waitingForPort.addCallback(stopIt)
        def stop(passthrough):
            self.running = False
            return passthrough
        d.addBoth(stop)
        return d

        Stop listening on the port if it is already listening, otherwise,
        cancel the attempt to listen.

        @return: a L{Deferred<twisted.internet.defer.Deferred>} which fires
            with L{None} when the port has stopped listening.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/internet.py