def test_addWriter(self):
        """
        Adding a writer when there was previously no writer starts up a
        C{LoopingCall}.
        """
        poller = _ContinuousPolling(Clock())
        self.assertIsNone(poller._loop)
        writer = object()
        self.assertFalse(poller.isWriting(writer))
        poller.addWriter(writer)
        self.assertIsNotNone(poller._loop)
        self.assertTrue(poller._loop.running)
        self.assertIs(poller._loop.clock, poller._reactor)
        self.assertTrue(poller.isWriting(writer))

        Adding a writer when there was previously no writer starts up a
        C{LoopingCall}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_epollreactor.py