def test_cantRegisterTwice(self):
        """
        It is not possible to register more than one C{Application}.
        """
        reactor = gireactor.GIReactor(useGtk=False)
        self.addCleanup(self.unbuildReactor, reactor)
        app = Gio.Application(
            application_id='com.twistedmatrix.trial.gireactor',
            flags=Gio.ApplicationFlags.FLAGS_NONE)
        reactor.registerGApplication(app)
        app2 = Gio.Application(
            application_id='com.twistedmatrix.trial.gireactor2',
            flags=Gio.ApplicationFlags.FLAGS_NONE)
        exc = self.assertRaises(RuntimeError,
                                    reactor.registerGApplication, app2)
        self.assertEqual(exc.args[0],
                         "Can't register more than one application instance.")

        It is not possible to register more than one C{Application}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_gireactor.py