Django Signals

Django Signals

What: Observer pattern. Something happens (e.g. model save) → receivers run. Decouples “who did it” from “what runs after.”

• Built-in model signals: pre_save, post_save, pre_delete, post_delete, m2m_changed. Use @receiver(post_save, sender=MyModel).
• Custom signal: from django.dispatch import Signal → my_signal = Signal(providing_args=['order']) → my_signal.send(sender=self, order=...). Receivers connect with @receiver(my_signal).
• Register receivers: In AppConfig.ready() do import myapp.signals so they’re loaded.

Use for: Creating a Profile when User is created, audit log on save/delete, cache invalidation, sending an email when Order is created. Keep handlers light — no heavy work; queue a Celery task if needed.

Gotchas: Recursion (e.g. post_save on User creates Profile, post_save on Profile updates User → infinite loop). Avoid saving the same model or the sender that triggered the signal inside the receiver. For tests: disconnect the receiver in setUp if you don’t want it, reconnect in tearDown.

Bulk ops: bulk_create / update() don’t fire model signals. Disconnect temporarily if you need to skip them, or run logic in a different path.