def test_makeConnection(self):
        """
        Test that the client and server protocol both have makeConnection
        invoked on them by loopbackAsync.
        """
        class TestProtocol(Protocol):
            transport = None
            def makeConnection(self, transport):
                self.transport = transport

        server = TestProtocol()
        client = TestProtocol()
        loopback.loopbackAsync(server, client)
        self.assertIsNotNone(client.transport)
        self.assertIsNotNone(server.transport)

        Test that the client and server protocol both have makeConnection
        invoked on them by loopbackAsync.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_loopback.py