id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
17029	django.dispatch.dispatcher.Signal should be pickleable	Dougal Matthews	nobody	"The Signal class contains an instance of threading.Lock which means it can't be pickled without providing pickle with some hints. It should be pickleable as it is passed in the kwargs to signal receivers, the use case that requires all arguments to be pickleable is best demonstrated as follows.

This example uses a celery task as the receiver of a signal to in effect provide ""async signals"" which would allow for some nice patterns to be developed.

{{{
# In a models.py file in any project with celery already configured.
from celery.task import task
from django.db import models
from django.db.models.signals import pre_delete, pre_save

class MyModel(models.Model):
    pass

@task(ignore_result=True)
def async_post_save(sender, instance, **kwargs):
    # do something with the instance.
    pass

post_save.connect(async_post_save.delay, sender=MyModel)
}}}


I've attached a patch for this which basically implements a very small monkey patch that I am currently using in a project. 

{{{
from django.dispatch.dispatcher import Signal
def reducer(self):
    return (Signal, (self.providing_args,))
Signal.__reduce__ = reducer
}}}


As it happens, I quickly threw together a blog post about this trick: http://dougalmatthews.com/2011/10/10/making-django's-signals-asynchronous-with-celery/"	Bug	closed	Core (Other)	1.3	Normal	wontfix			Unreviewed	1	0	0	0	1	0