LoopingCall AMP commands

Question

Why do I get an error while trying to implement LoopingCall function calling AMP commands?

from twisted.protocols.amp import AMP
from twisted.python.log import startLogging, err
from twisted.internet.task import LoopingCall
from twisted.internet import reactor
from sys import stdout

import commands

startLogging(stdout)


class MyAMP:

    def __init__(self, host, port):
        destination = TCP4ClientEndpoint(reactor, host, port)
        self.protocol = AMP()
        self.d = connectProtocol(destination, self.protocol)

    def say(self):
        return self.protocol.callRemote(commands.Say,
                                   phrase='Hello world')


def loop(myamp):
    myamp.say()


def main(host, port):
    myamp = MyAMP(host, port)
    lc = LoopingCall(loop, myamp=myamp)
    lc.start(4.0)
    reactor.run()

main('127.0.0.1', 12345)

Error while calling myamp.say() within loop:

2013-08-16 12:28:58-0400 [-] Starting factory <twisted.internet.endpoints.OneShotFactory instance at 0x92273ec>
2013-08-16 12:28:58-0400 [-] Unhandled error in Deferred:
2013-08-16 12:28:58-0400 [-] Unhandled Error
    Traceback (most recent call last):
      File "lib/client.py", line 35, in <module>
        main('127.0.0.1', 12345)
      File "lib/client.py", line 32, in main
        lc.start(4.0)
      File "/usr/local/lib/python2.7/site-packages/twisted/internet/task.py", line 173, in start
        self()
      File "/usr/local/lib/python2.7/site-packages/twisted/internet/task.py", line 218, in __call__
        d = defer.maybeDeferred(self.f, *self.a, **self.kw)
    --- <exception caught here> ---
      File "/usr/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 137, in maybeDeferred
        result = f(*args, **kw)
      File "lib/client.py", line 26, in loop
        myamp.say()
      File "lib/client.py", line 22, in say
        phrase='Hello world')
      File "/usr/local/lib/python2.7/site-packages/twisted/protocols/amp.py", line 821, in callRemote
        return co._doCommand(self)
      File "/usr/local/lib/python2.7/site-packages/twisted/protocols/amp.py", line 1778, in _doCommand
        self.requiresAnswer)
      File "/usr/local/lib/python2.7/site-packages/twisted/protocols/amp.py", line 752, in _sendBoxCommand
        box._sendTo(self.boxSender)
      File "/usr/local/lib/python2.7/site-packages/twisted/protocols/amp.py", line 577, in _sendTo
        proto.sendBox(self)
    exceptions.AttributeError: 'NoneType' object has no attribute 'sendBox'

2013-08-16 12:28:58-0400 [Uninitialized] AMP connection established (HOST:IPv4Address(TCP, '127.0.0.1', 50457) PEER:IPv4Address(TCP, '127.0.0.1', 12345))

Answer 1

You're trying to callRemote before the connection is established. A LoopingCall will, by default, immediately run its function when you start it. Instead of doing lc.start(4.0), do lc.start(4.0, now=False). This will wait four second before the first call.

Answer 2

In the normal environment, where the network connection is rock stable, the way of @habnabit will be working, but in the real world, the connection latency can not be estimated as you expect. The better solution for this problem, the looping call must be executed after the amp client is connected like this.

from twisted.protocols.amp import AMP
from twisted.python.log import startLogging, err
from twisted.internet.task import LoopingCall
from twisted.internet import reactor, endpoints
from sys import stdout

import commands

startLogging(stdout)


class MyAMP:
    def __init__(self, host, port):
        destination = endpoints.TCP4ClientEndpoint(reactor, host, port)
        self.protocol = AMP()
        self.d = endpoints.connectProtocol(destination, self.protocol)


def loop (proto, ) :
    return proto.callRemote(commands.get_user, key='Hello world')

def main(host, port):
    def _cb_connected (proto, ) :
        lc = LoopingCall(loop, proto, )
        lc.start(4.0)
        return

    myamp = MyAMP(host, port)
    myamp.d.addCallback(_cb_connected, )
    reactor.run()

    return

main('127.0.0.1', 12345, )