How do I add an errback to deferLater?

Question

Consider the following twisted code, using deferLater:

import random

from twisted.internet.task import deferLater
from twisted.internet import reactor

def random_exception(msg='general'):
    if random.random() < 0.5:
        raise Exception("Random exception with 50%% likelihood occurred in %s!" % msg)

def dolater():
    random_exception('dolater')
    print "it's later!"

def whoops(failure):
    failure.trap(Exception)
    print failure

defer = deferLater(reactor, 10, dolater)
defer.addErrback(whoops)

reactor.run()

An exception is raised during the 10 second sleep (namely a KeyboardInterrupt), however, it seems that the whoops method is never called. My assumption is that since I add the errBack after the deferred kicks off, it's never properly registered. Advice appreciated.

EDIT:

Alright, no one likes my use of the signal (not the exception) KeyboardInterrupt to show an error condition outside of the defer. I thought pretty hard about an actual exception that might occur out of the defer callback, but couldn't think of a particularly good one, most everything would be some kind of signal (or developer error), so signal handling is fine for now- but that wasn't really the heart of the question.

As I understand it, twisted's callback/errback system handles errors within the callback structure - e.g. if dolater raises an Exception of some kind. To show this, I have added an exception that could occur during dolater, to show that if the exception occurs in dolater, the errback handles the exception just fine.

My concern was if something went wrong while the reactor was just reacting normally, and the only thing I could get to go wrong was a keyboard interrupt, then I wanted whoops to fire. It appears that if I put other async events into the reactor and raise exceptions from there, then the dolater code wouldn't be affected, and I would have to add errbacks to those other async events. There is no master error handling for an entire twisted program.

So signals it is, until I can find some way to cause the reactor to fail without a signal.

Answer 1

If by KeyboardInterrupt you mean a signal (ctrl-c, SIGINT, etc), then what you need to do is setup a signal handler with your whoops function as the callback.

By following two previous answers from @jean-paul-calderone twisted: catch keyboardinterrupt and shutdown properly and twisted - interrupt callback via KeyboardInterrupt, I tried the following, and I think it matches your need:

def dolater():
    print "it's later!"

def whoops(signal, stackframe):
    print "I'm here because of signal number " + str(signal)
    reactor.stop()

defer = task.deferLater(reactor, 10, dolater)
signal.signal(signal.SIGINT, whoops)
reactor.run()

That will call whoops on a SIGINT. I put a reactor.stop() in the whoops because otherwise the reactor would just keep on running, take that out if you really want it to keep running in the face of a ctrl-c.

Note: I'm not explicitly showing how to fire a err-back in the signal system because (at least to my understanding) that doesn't really map to how defer'ed should be used. I imagine if you found a way to get the defer'ed into the signal handler you could fire its errback but I think thats out of the expected use-case for twisted and may have crazy consequences.