def _greetingtest(self, write, testServer):
        """
        Test one of the permutations of write/writeSequence client/server.

        @param write: The name of the method to test, C{"write"} or
            C{"writeSequence"}.
        """
        class GreeteeProtocol(Protocol):
            bytes = b""
            def dataReceived(self, bytes):
                self.bytes += bytes
                if self.bytes == b"bytes":
                    self.received.callback(None)

        class GreeterProtocol(Protocol):
            def connectionMade(self):
                if write == "write":
                    self.transport.write(b"bytes")
                else:
                    self.transport.writeSequence([b"byt", b"es"])

        if testServer:
            server = GreeterProtocol()
            client = GreeteeProtocol()
            d = client.received = Deferred()
        else:
            server = GreeteeProtocol()
            d = server.received = Deferred()
            client = GreeterProtocol()

        loopback.loopbackAsync(server, client)
        return d

        Test one of the permutations of write/writeSequence client/server.

        @param write: The name of the method to test, C{"write"} or
            C{"writeSequence"}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_loopback.py