python twisted json decoding

Question

I've got a following code developed using python twisted library:

class Cache(protocol.Protocol):
    def __init__(self, factory):
        self.factory = factory

    def dataReceived(self, data):
        request = json.loads(data)
        self.factory.handle[request['command']](**request)
        self.transport.write(data)

class CacheFactory(protocol.Factory):
    def buildProtocol(self, addr):
        return Cache(self)
    def handle_get(self, **kwargs):
        print 'get\n', kwargs
    def handle_set(self, **kwargs):
        print 'set\n', kwargs
    def handle_delete(self, **kwargs):
        print 'delete\n', kwargs
    handle = {
        'get': handle_get,
        'set': handle_set,
        'delete': handle_delete,
    }

reactor.listenTCP(int(sys.argv[1]), CacheFactory())
reactor.run()

I run a client connection using telnet:

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
{"command": "set", "value": 1234567890}
Connection closed by foreign host.

an exception is thrown:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/python/log.py", line 84, in callWithLogger
    return callWithContext({"system": lp}, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/log.py", line 69, in callWithContext
    return context.call({ILogContext: newCtx}, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/internet/selectreactor.py", line 146, in _doReadOrWrite
    why = getattr(selectable, method)()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 460, in doRead
    rval = self.protocol.dataReceived(data)
  File "./server.py", line 18, in dataReceived
    self.factory.handle[request['command']](**request)
exceptions.TypeError: handle_set() takes exactly 1 argument (0 given)

which I don't understand. There could be something wrong with line self.factory.handle[request['command']](**request) , but in my opinion it's correct - it passes the self parameter implicitly (it's a method afterall) and unpacked request parameter explicitly. The exception message says the function takes 1 argument, and it's a lie :) because it takes 2 parameters: self, **kwargs . And it's not true that I pass 0 arguments, since I pass 2.

Can someone help me to spot the problem?

---

in case it helps, the json request is decoded into:

{u'command': u'set', u'value': 1234567890}

Answer 1

As it is now, the handle_* methods are instance methods, yet the handle dict points to unbound methods. That is, self is not being passed implicitly. Try this instead:

class CacheFactory(protocol.Factory):
    def buildProtocol(self, addr):
        return Cache(self)
    def handle_get(self, **kwargs):
        print 'get\n', kwargs
    def handle_set(self, **kwargs):
        print 'set\n', kwargs
    def handle_delete(self, **kwargs):
        print 'delete\n', kwargs
    def __init__(self, *args, **kwargs):
        protocol.Factory.__init__(self, *args, **kwargs)
        self.handle = {
            'get': self.handle_get,
            'set': self.handle_set,
            'delete': self.handle_delete,
        }

Alternatively, you could keep your handle the same and do this:

    def dataReceived(self, data):
        request = json.loads(data)
        self.factory.handle[request['command']](self.factory, **request)
        self.transport.write(data)

Or, you could take this approach and then you don't need a handle dict either way:

    def dataReceived(self, data):
        request = json.loads(data)
        getattr(self.factory, "handle_%s" % (request['command'],))(**request)
        self.transport.write(data)

Also note your dataReceived , as it is now, is unsafe, because packets might get split up arbitrarily - that is, you might not receive an entire json message in one shot.