def _hostpeertest(self, get, testServer):
        """
        Test one of the permutations of client/server host/peer.
        """
        class TestProtocol(Protocol):
            def makeConnection(self, transport):
                Protocol.makeConnection(self, transport)
                self.onConnection.callback(transport)

        if testServer:
            server = TestProtocol()
            d = server.onConnection = Deferred()
            client = Protocol()
        else:
            server = Protocol()
            client = TestProtocol()
            d = client.onConnection = Deferred()

        loopback.loopbackAsync(server, client)

        def connected(transport):
            host = getattr(transport, get)()
            self.assertTrue(IAddress.providedBy(host))

        return d.addCallback(connected)

        Test one of the permutations of client/server host/peer.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_loopback.py