CODEWITHSUNDEEP
pythontwistedperspective-broker
Deano123
Deano123

Reputation: 161

Struggling to understand Twisted in general and pb in particular

Could someone explain the difference between the following please. I am really struggling to grasp Deferred concept, I thought I had it as I have been doing examples all day. But I think i must be code blind. I'm sure its really simple.

This works.

from twisted.spread import pb
from twisted.internet import reactor
from twisted.python import util


if __name__ == '__main__':

    def print_result(result):
        print result

    def add_numbers(obj, a, b):
        obj.callRemote("add_numbers", a, b)



    factory = pb.PBClientFactory()
    reactor.connectTCP("localhost", 8789, factory)
    d = factory.getRootObject()
    d.addCallback(lambda object: object.callRemote("add_numbers", 1, 2))
    d.addErrback(lambda reason: 'error: '+str(reason.value))
    d.addCallback(print_result)
    d.addErrback(lambda reason: 'error: '+str(reason.value))
    d.addCallback(lambda _: reactor.stop())
    d = factory.getRootObject()

    reactor.run()

and this doesnt

from twisted.spread import pb
from twisted.internet import reactor
from twisted.python import util


if __name__ == '__main__':

    def print_result(result):
        print result

    def add_numbers(obj, a, b):
        obj.callRemote("add_numbers", a, b)



    factory = pb.PBClientFactory()
    reactor.connectTCP("localhost", 8789, factory)
    d = factory.getRootObject()
    d.addCallback(add_numbers, 1, 2)
    d.addErrback(lambda reason: 'error: '+str(reason.value))
    d.addCallback(print_result)
    d.addErrback(lambda reason: 'error: '+str(reason.value))
    d.addCallback(lambda _: reactor.stop())
    d = factory.getRootObject()

    reactor.run()

I cant for the life of me work out why, it crashes with this error:

Unhandled error in Deferred:
Unhandled Error
Traceback (most recent call last):
Failure: twisted.spread.pb.PBConnectionLost: [Failure instance: Traceback (failure  with no frames): <class 'twisted.internet.error.ConnectionLost'>: Connection to the other side was lost in a non-clean fashion: Connection lost.
]

Server side code is

from twisted.spread import pb
from twisted.internet import reactor

class Echoer(pb.Root):
    def remote_echo(self, st):
        print 'echoing:', st
        return st

    def remote_add_numbers(self, a, b):
        print 'adding:', a, b
        c = a + b
        return c

if __name__ == '__main__':
    reactor.listenTCP(8789, pb.PBServerFactory(Echoer()))
    reactor.run()

Upvotes: 2

Views: 460

Answers (1)

Glyph
Glyph

Reputation: 31910

The difference between your working and broken examples is that the lambda expression implicitly returns its result. Its result is a Deferred, which means that the next callback in the chain will wait to execute until its result is available.

If you change the definition of add_numbers to return the result of callRemote, like so:

def add_numbers(obj, a, b):
    return obj.callRemote("add_numbers", a, b)

Then your broken example will start working again.

Upvotes: 2

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