Making Django's Signals Async with Celery

Update: A comment on the ticket I opened by Alex Gaynor brought up a point that I hadn't fully considered. It's worth noticing before going further in this post and also worth pointing out my monkey patch doesn't answer this question.

After speaking with Carl, I'm marking this as wontfix because it is non-obvious as to whether pickling a Signal should include the registered receivers, and how that interacts with the weak referencing, since there's no obvious semantic it seems better not to guess.

Continue on if you want to see how I did it and may be willing to live with a monkey patch...

I really enjoy working with both Django's signal framework and Celery tasks. Today it occured to me that it would be useful to combine the two and have “asynchronous signals”.

Here is the solution that I came up with, read on below if you want to see how I arrived at this and why we need to monkey patch.

from celery.task import task
from django.db.models.signals import post_save

from myproject.models import MyModel

# Warning. Monkey patch.
from django.dispatch.dispatcher import Signal
def reducer(self):
    return (Signal, (self.providing_args,))
Signal.__reduce__ = reducer

# With the patch done, we can now connect to celery tasks.
@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)

The first solution that occured to me was to use an intermediate function that triggered the task. This works fine and doesn't require anything clever.

from celery.task import task
from django.db.models.signals import post_save

from myproject.models import MyModel

@task
def async_post_save(instance):
    # do something with the instance.
    pass

def post_save_reciever(sender, instance, **kwargs):
    async_post_save.delay()
post_save.connect(post_save_reciever)

However, this adds an extra level of redirection to code that shouldn't be needed. Why can't be connect to tasks directly?

@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)

This almost works, however, in the kwargs signal recievers are passed an instance of django.display.dispatcher.Signal and this contains an instance of threading.Lock - an object that can't be pickled. This leads me to the monkey patch that was shown at the start of this article which simply adds a __reduce__ method to the Signal class that alters the pickle behaviour and only pickles the provided_args property of the Signal instance.

Incidently, you'll notice that I added ignore_result=True to each of the tasks. While this isn't required, its not generally standard practice for signals recievers to return anything, so you will probably want to do this too.

View the ticket I opened a ticket to track this idea This link