What are Django signals and when to use them?

Question

Accepted Answer

**Django signals** allow **decoupled communication** between components when actions occur.

```python
from django.db.models.signals import post_save, pre_delete
from django.dispatch import receiver
from django.contrib.auth.models import User

# Signal handler
@receiver(post_save, sender=User)
def create_profile(sender, instance, created, **kwargs):
    if created:
        Profile.objects.create(user=instance)
        print(f"Profile created for {instance.username}")

@receiver(pre_delete, sender=User)
def cleanup_data(sender, instance, **kwargs):
    # Clean up related data before deletion
    instance.profile.delete()
    instance.posts.all().delete()

# Custom signals
from django.dispatch import Signal

order_completed = Signal()  # Signal definition

# Sending signal
order_completed.send(
    sender=Order,
    order_id=order.id,
    total=order.total
)

# Connecting handler
order_completed.connect(send_confirmation_email)
```

**Built-in signals:**
- `pre_save` / `post_save`.
- `pre_delete` / `post_delete`.
- `m2m_changed`.
- `request_started` / `request_finished`.

**When to use:**
- Cross-app communication.
- Side effects on model changes.
- Audit logging.

Python: What are Django signals and when to use them?