def test_removedFromReactor(self):
        """
        A descriptor's C{fileno} method should not be called after the
        descriptor has been removed from the reactor.
        """
        reactor = self.buildReactor()
        descriptor = RemovingDescriptor(reactor)
        reactor.callWhenRunning(descriptor.start)
        self.runReactor(reactor)
        self.assertEqual(descriptor.calls, [])

        A descriptor's C{fileno} method should not be called after the
        descriptor has been removed from the reactor.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_fdset.py