def test_synchronousStop(self):
        """
        L{task.react} handles when the reactor is stopped just before the
        returned L{Deferred} fires.
        """
        def main(reactor):
            d = defer.Deferred()
            def stop():
                reactor.stop()
                d.callback(None)
            reactor.callWhenRunning(stop)
            return d
        r = _FakeReactor()
        exitError = self.assertRaises(
            SystemExit, task.react, main, [], _reactor=r)
        self.assertEqual(0, exitError.code)

        L{task.react} handles when the reactor is stopped just before the
        returned L{Deferred} fires.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_task.py