Why doesn't this asyncore.dispatcher.handle_read() get called?

Jean-Paul Calderone calderone.jeanpaul at gmail.com
Wed Apr 20 22:01:08 CEST 2011

On Apr 20, 12:25 pm, Dun Peal <dunpea... at gmail.com> wrote:
> Hi,
> I'm writing and testing an asyncore-based server. Unfortunately, it
> doesn't seem to work. The code below is based on the official docs and
> examples, and starts a listening and sending dispatcher, where the
> sending dispatcher connects and sends a message to the listener - yet
> Handler.handle_read() never gets called, and I'm not sure why. Any
> ideas?
> Thanks, D.
> import asyncore, socket, sys
> COMM_PORT = 9345
> class Handler(asyncore.dispatcher):
>     def handle_read(self):
>         print 'This never prints'
> class Listener(asyncore.dispatcher):
>     def __init__(self, port=COMM_PORT):
>         asyncore.dispatcher.__init__(self)
>         self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
>         self.set_reuse_addr()
>         self.bind(('', port))
>         self.listen(5)
>     def handle_accept(self):
>         client, addr = self.accept()
>         print 'This prints.'
>         return Handler(client)
> class Sender(asyncore.dispatcher):
>     def __init__(self, host):
>         asyncore.dispatcher.__init__(self)
>         self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
>         self.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
>         self.connect( (host, COMM_PORT) )
>         self.buffer = 'Msg\r\n'
>     def handle_connect(self):
>         pass
>     def writable(self):
>         return len(self.buffer) > 0
>     def handle_write(self):
>         sent = self.send(self.buffer)
>         self.buffer = self.buffer[sent:]
> def test_communication():
>     from multiprocessing import Process
>     def listener():
>         l = Listener()
>         asyncore.loop(timeout=10, count=1)
>     lis = Process(target=listener)
>     lis.start()
>     def sender():
>         s = Sender('localhost')
>         asyncore.loop(timeout=10, count=1)
>     sen = Process(target=sender)
>     sen.start()
>     lis.join()
> test_communication()

You didn't let the program run long enough for the later events to
happen.  loop(count=1) basically means one I/O event will be processed
- in the case of your example, that's an accept().  Then asyncore is
done and it never gets to your custom handle_read.

So you can try passing a higher count to loop, or you can add your own
loop around the loop call.  Or you can switch to Twisted which
actually makes testing a lot easier than this - no need to spawn
multiple processes or call accept or recv yourself.  Here's a somewhat
equivalent Twisted-based version of your program:

    from twisted.internet.protocol import ServerFactory, Protocol
    from twisted.internet import reactor

    factory = ServerFactory()
    factory.protocol = Protocol

    reactor.listenTCP(0, factory)

It's hard to write the equivalent unit test, because the test you
wrote for the asyncore-based version is testing lots of low level
details which, as you can see, don't actually appear in the Twisted-
based version because Twisted does them for you already.  However,
once you get past all that low-level stuff and get to the part where
you actually implement some of your application logic, you might have
tests for your protocol implementation that look something like this:

    from twisted.trial.unittest import TestCase
    from twisted.test.proto_helpers import StringTransport

    from yourapp import Handler # Or a better name

    class HandlerTests(TestCase):
        def test_someMessage(self):
            When the "X" message is received, the "Y" response is sent
            transport = StringTransport()
            protocol = Handler()
            self.assertEqual(transport.value(), "Y")

Hope this helps,

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