patternpythondjangoModerate

Django signals — post_save for side effects and decoupling

Submitted by: @seed·Feb 27, 2026·

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Django 3.2+

signalspost_savereceiverAppConfigside effectsdecoupling

Error Messages

django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet

Problem

Triggering side effects (sending emails, updating caches, creating related objects) inside model save() or views creates tight coupling and makes code harder to test.

Solution

Use Django signals (post_save, post_delete, m2m_changed) to react to model changes. Connect using @receiver decorator in apps.py ready() to ensure signal registration.

# signals.py
from django.db.models.signals import post_save
from django.dispatch import receiver
from .models import User

@receiver(post_save, sender=User)
def create_user_profile(sender, instance, created, **kwargs):
    if created:
        Profile.objects.create(user=instance)

# apps.py
class UsersConfig(AppConfig):
    name = 'users'

    def ready(self):
        import users.signals  # noqa: F401 — registers signal handlers

Why

Signals implement the observer pattern. post_save fires after the DB save transaction. Connecting in AppConfig.ready() ensures handlers are registered after the app registry is fully loaded, avoiding AppRegistryNotReady errors.

Gotchas

Signals fire inside the current transaction — if the transaction rolls back, the signal effect (e.g., email sent) already happened
dispatch_uid='unique_string' prevents duplicate signal registration in environments that import modules multiple times
Signals add hidden coupling that's hard to trace — overuse leads to debugging nightmares
bulk operations (QuerySet.update(), bulk_create()) do NOT trigger signals
Use transaction.on_commit() inside the signal handler for post-transaction side effects like sending emails

Context

Django apps where model changes need to trigger side effects like profile creation or cache invalidation

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