Django Signals

Built-in Signal Types

@receiver(pre_save, sender=Article)
def before_save(sender, instance, **kwargs):
    instance.slug = slugify(instance.title)

@receiver(post_save, sender=Order)
def after_save(sender, instance, created, **kwargs):
    if created:
        send_confirmation_email(instance)

@receiver(pre_delete, sender=Profile)
def before_delete(sender, instance, **kwargs):
    instance.avatar.delete()

@receiver(m2m_changed, sender=User.groups.through)
def groups_changed(sender, instance, **kwargs):
    print(f"Groups updated for {instance}")

🔹 Creating Signal Receivers

Signal receivers are functions that automatically execute when specific events occur. They help you add functionality without modifying existing code, keeping your application modular and maintainable.

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

@receiver(post_save, sender=User)
def create_user_profile(sender, instance, created, **kwargs):
    """Create a profile when a new user is created"""
    if created:
        Profile.objects.create(user=instance)
        print(f"Profile created for {instance.username}")

@receiver(post_save, sender=User)
def save_user_profile(sender, instance, **kwargs):
    """Save profile when user is saved"""
    instance.profile.save()
    print(f"Profile saved for {instance.username}")

🔹 Registering Signals

Signals must be imported when Django starts. The best practice is to import them in your app's apps.py ready() method to ensure they're loaded properly.

# apps.py
from django.apps import AppConfig

class MyAppConfig(AppConfig):
    default_auto_field = 'django.db.models.BigAutoField'
    name = 'myapp'
    
    def ready(self):
        import myapp.signals  # Import signals here

# __init__.py
default_app_config = 'myapp.apps.MyAppConfig'

🔹 Custom Signals

You can create your own custom signals for application-specific events. This is useful for creating loosely coupled components that react to custom business logic events.

# signals.py
from django.dispatch import Signal, receiver

# Define custom signal
order_placed = Signal()

# Send signal from view
def place_order(request):
    order = Order.objects.create(user=request.user)
    order_placed.send(sender=Order, order=order, user=request.user)
    return HttpResponse("Order placed!")

# Receive custom signal
@receiver(order_placed)
def send_order_notification(sender, order, user, **kwargs):
    print(f"Order #{order.id} placed by {user.username}")
    # Send email, update inventory, etc.

🔹 Practical Example: Email Notifications

This example shows how to automatically send welcome emails when users register and notification emails when they make purchases.

# signals.py
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.core.mail import send_mail
from django.contrib.auth.models import User
from .models import Order

@receiver(post_save, sender=User)
def send_welcome_email(sender, instance, created, **kwargs):
    if created:
        send_mail(
            subject='Welcome to Our Site!',
            message=f'Hi {instance.username}, thanks for joining!',
            from_email='[email protected]',
            recipient_list=[instance.email],
        )
        print(f"Welcome email sent to {instance.email}")

@receiver(post_save, sender=Order)
def send_order_confirmation(sender, instance, created, **kwargs):
    if created:
        send_mail(
            subject='Order Confirmation',
            message=f'Your order #{instance.id} has been received!',
            from_email='[email protected]',
            recipient_list=[instance.user.email],
        )
        print(f"Order confirmation sent for Order #{instance.id}")

🔹 Signal Best Practices

Follow these guidelines to use signals effectively: