def test_stopServiceOnStoppedService(self):
        """
        Calling L{ClientService.stopService} on a stopped service
        returns a L{Deferred} that has already fired with L{None}.
        """
        clock = Clock()
        _, service = self.makeReconnector(fireImmediately=False,
                                          clock=clock)
        firstStopDeferred = service.stopService()
        secondStopDeferred = service.stopService()

        self.assertIsNone(self.successResultOf(firstStopDeferred))
        self.assertIsNone(self.successResultOf(secondStopDeferred))

        Calling L{ClientService.stopService} on a stopped service
        returns a L{Deferred} that has already fired with L{None}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/test/test_internet.py