def test_writeNotReentrant(self):
        """
        L{loopback.loopbackAsync} does not call a protocol's C{dataReceived}
        method while that protocol's transport's C{write} method is higher up
        on the stack.
        """
        class Server(Protocol):
            def dataReceived(self, bytes):
                self.transport.write(b"bytes")

        class Client(Protocol):
            ready = False

            def connectionMade(self):
                reactor.callLater(0, self.go)

            def go(self):
                self.transport.write(b"foo")
                self.ready = True

            def dataReceived(self, bytes):
                self.wasReady = self.ready
                self.transport.loseConnection()

        server = Server()
        client = Client()
        d = loopback.loopbackAsync(client, server)
        def cbFinished(ignored):
            self.assertTrue(client.wasReady)
        d.addCallback(cbFinished)
        return d

        L{loopback.loopbackAsync} does not call a protocol's C{dataReceived}
        method while that protocol's transport's C{write} method is higher up
        on the stack.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_loopback.py