def test_registerProducers(self):
        """
        The proxy client registers itself as a producer of the proxy server and
        vice versa.
        """
        # create a ProxyServer instance
        addr = address.IPv4Address('TCP', '127.0.0.1', 0)
        server = portforward.ProxyFactory('127.0.0.1', 0).buildProtocol(addr)

        # set the reactor for this test
        reactor = proto_helpers.MemoryReactor()
        server.reactor = reactor

        # make the connection
        serverTransport = proto_helpers.StringTransport()
        server.makeConnection(serverTransport)

        # check that the ProxyClientFactory is connecting to the backend
        self.assertEqual(len(reactor.tcpClients), 1)
        # get the factory instance and check it's the one we expect
        host, port, clientFactory, timeout, _ = reactor.tcpClients[0]
        self.assertIsInstance(clientFactory, portforward.ProxyClientFactory)

        # Connect it
        client = clientFactory.buildProtocol(addr)
        clientTransport = proto_helpers.StringTransport()
        client.makeConnection(clientTransport)

        # check that the producers are registered
        self.assertIs(clientTransport.producer, serverTransport)
        self.assertIs(serverTransport.producer, clientTransport)
        # check the streaming attribute in both transports
        self.assertTrue(clientTransport.streaming)
        self.assertTrue(serverTransport.streaming)

        The proxy client registers itself as a producer of the proxy server and
        vice versa.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_protocols.py