def test_cantRegisterAfterRun(self):
        """
        It is not possible to register a C{Application} after the reactor has
        already started.
        """
        reactor = gireactor.GIReactor(useGtk=False)
        self.addCleanup(self.unbuildReactor, reactor)
        app = Gio.Application(
            application_id='com.twistedmatrix.trial.gireactor',
            flags=Gio.ApplicationFlags.FLAGS_NONE)

        def tryRegister():
            exc = self.assertRaises(ReactorAlreadyRunning,
                                    reactor.registerGApplication, app)
            self.assertEqual(exc.args[0],
                             "Can't register application after reactor was started.")
            reactor.stop()
        reactor.callLater(0, tryRegister)
        ReactorBuilder.runReactor(self, reactor)

        It is not possible to register a C{Application} after the reactor has
        already started.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_gireactor.py