django-q-signals

Process Django signals asynchronously with Django Q2.

Requirements

• Python 3.10, 3.11, 3.12, 3.13
• Django 4.2, 5.1, 5.2

Installation

1. Install the package from PyPI.

   python -m pip install django-q-signals
   
   # or if you like the new hotness
   
   uv add django-q-signals
   uv sync

2. Ensure Django Q2 is installed and correctly configured for your project.

Getting Started

Django signals run synchronously by default, potentially blocking your request/response cycle while they process. Even async handlers, which Django's signals support, still block the request:

from django.db.models.signals import post_save
from django.dispatch import receiver

from myapp.models import Article


@receiver(post_save, sender=Article)
async def notify_subscribers(sender, instance, **kwargs):
    # This async handler STILL blocks the request!
    for subscriber in instance.subscribers.all():
        await send_email_async(subscriber.email, f"New article: {instance.title}")

    # More blocking async operations
    await update_search_index_async(instance)
    await generate_thumbnails_async(instance)
    await ping_external_apis_async(instance)

You could manually offload this to Django Q2's task queue:

from django.db.models.signals import post_save
from django.dispatch import receiver
from django_q.tasks import async_task

from myapp.models import Article


def process_article_task(article_id):
    article = Article.objects.get(pk=article_id)
    for subscriber in article.subscribers.all():
        send_email(subscriber.email, f"New article: {article.title}")
    update_search_index(article)
    generate_thumbnails(article)
    ping_external_apis(article)


@receiver(post_save, sender=Article)
def notify_subscribers(sender, instance, **kwargs):
    # Queue the task instead of running it now
    async_task('myapp.tasks.process_article_task', instance.pk)

But who wants to write all that boilerplate every time you need to offload a signal handler? (Yes, it's barely any boilerplate and the explicit version is arguably clearer, but let me have this.)

Instead, @async_receiver can handle all this for you:

from django.db.models.signals import post_save
from django_q_signals import async_receiver

from myapp.models import Article


@async_receiver(post_save, sender=Article)
def notify_subscribers(sender, instance, **kwargs):
    # This automatically runs in the background via Django Q2
    for subscriber in instance.subscribers.all():
        send_email(subscriber.email, f"New article: {instance.title}")
    update_search_index(instance)
    generate_thumbnails(instance)
    ping_external_apis(instance)

Your views return immediately while the heavy lifting happens in the background.

Usage

The @async_receiver decorator transforms your signal handler into an asynchronous task by:

1. Creating a serializable wrapper function that Django Q2 can pickle
2. Registering that wrapper as a module-level function for Django Q2 to import
3. Intercepting signals and queuing tasks with serialized instance data
4. Reconstructing model instances from their primary keys when the task runs

This means your handler receives the same arguments as a normal signal handler, but runs in Django Q2's task queue instead of blocking the request.

The @async_receiver decorator accepts the same arguments as Django's @receiver:

• signal: One or more signals to connect to
• sender: Optional model class to filter signals
• **kwargs: Additional options like dispatch_uid for preventing duplicates

# Prevent duplicate registrations with dispatch_uid
@async_receiver(post_save, sender=Article, dispatch_uid="unique_article_handler")
def process_article(sender, instance, **kwargs):
    generate_thumbnails(instance)

# Handle multiple signals with one handler
@async_receiver([post_save, post_delete], sender=Article)
def update_search_index(sender, instance, **kwargs):
    if kwargs.get('created', False):
        add_to_index(instance)
    elif instance is None:
        # Instance was deleted but pk is preserved
        if pk := kwargs.get('_instance_pk'):
            remove_from_index_by_id(pk)

Just like Django's @receiver decorator, @async_receiver supports async handlers:

@async_receiver(post_save, sender=Article)
async def process_article_async(sender, instance, **kwargs):
    await external_api.notify(instance.id)
    await cache.invalidate(f"article_{instance.id}")
    return await generate_summary(instance.content)

Async handlers are automatically wrapped with asgiref.sync.async_to_sync for execution in Django Q2's worker processes, matching Django's @receiver behavior.

Django Q2 Models

Django Q2's internal models are automatically excluded from async processing and cannot be used with the @async_receiver, to prevent infinite recursion. If you need to respond to Django Q2 model changes, use Django Q2's own signals or handle them manually using Django's standard @receiver decorator.

Serialization

Model instances are serialized by their primary key and reconstructed when the task runs.

The update_fields parameter is converted to a list internally before passing to Django Q2's async_task, then converted back to a frozenset before passing through to your handler, maintaining Django's behavior.

Only serializable signal kwargs (strings, numbers, lists, dicts, etc.) are passed to the async handler

Race Conditions

Since signal handlers decorated with @async_receiver run in a task queue asynchronously, there's a potential race condition where an instance might be deleted between when the signal fires and when the async task executes.

When an instance cannot be found during task execution, None is passed to your handler. However, the instance's primary key is preserved in kwargs['_instance_pk'] so you can still identify which object was affected:

@async_receiver(post_save, sender=Article)
def process_article(sender, instance, **kwargs):
    if instance is None:
        # Instance was deleted, but we have the pk
        article_id = kwargs.get('_instance_pk')
        logger.warning(f"Article {article_id} was deleted before processing")
        # Could still do cleanup based on the ID
        cleanup_article_artifacts(article_id)
        return

    # Normal processing
    generate_thumbnail(instance)

In particular, post_save signals with created=False (updates to existing instances) and m2m_changed signals are more prone to this race condition, since these often involve instances that might be deleted soon after modification. Newly created instances (created=True) are less likely to be immediately deleted, and for delete signals the instance is expected to be gone anyway.

Honestly, if you need a task to run regardless of instance deletion, you're better off using Django's built-in @receiver and calling async_task directly with the data you need, rather than using @async_receiver.

Development

For detailed instructions on setting up a development environment and contributing to this project, see CONTRIBUTING.md.

For release procedures, see RELEASING.md.

License

django-q-signals is licensed under the MIT license. See the LICENSE file for more information.