def test_cannotBind(self):
        """
        L{IReactorTCP.listenTCP} raises L{error.CannotListenError} if the
        address to listen on is already in use.
        """
        f = MyServerFactory()

        p1 = reactor.listenTCP(0, f, interface='127.0.0.1')
        self.addCleanup(p1.stopListening)
        n = p1.getHost().port
        dest = p1.getHost()
        self.assertEqual(dest.type, "TCP")
        self.assertEqual(dest.host, "127.0.0.1")
        self.assertEqual(dest.port, n)

        # make sure new listen raises error
        self.assertRaises(error.CannotListenError,
                          reactor.listenTCP, n, f, interface='127.0.0.1')

        L{IReactorTCP.listenTCP} raises L{error.CannotListenError} if the
        address to listen on is already in use.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_tcp.py