Receiver decorator in Django

Introduction

The receiver decorator helps connect a function to a signal in Django. It makes your function run automatically when something happens.

You want to run code right after a user signs up.

You need to update related data when a model is saved.

You want to send a notification when a comment is posted.

You want to log actions automatically when certain events happen.

Syntax

Django

@receiver(signal, sender=SenderModel)
def function_name(sender, instance, **kwargs):
    # your code here

Use @receiver above your function to connect it to a signal.

The sender argument limits the signal to a specific model or source.

Examples

This runs my_handler after MyModel is saved.

Django

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

@receiver(post_save, sender=MyModel)
def my_handler(sender, instance, **kwargs):
    print(f"Saved: {instance}")

This runs migration_handler after migrations finish.

Django

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

@receiver(post_migrate)
def migration_handler(sender, **kwargs):
    print("Migration finished!")

Sample Program

This example prints a message every time a Book is saved.

Django

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

class Book(models.Model):
    title = models.CharField(max_length=100)

@receiver(post_save, sender=Book)
def announce_book_saved(sender, instance, **kwargs):
    print(f"Book saved: {instance.title}")

Important Notes

Remember to import receiver from django.dispatch.

Signals can slow down your app if overused, so use them wisely.

Summary

The receiver decorator links a function to a Django signal.

It helps run code automatically when events happen.

Use sender to specify which model or source triggers the function.

Practice

1. What is the main purpose of the @receiver decorator in Django?

easy

A. To register a template filter

B. To create a new database model

C. To connect a function to a signal so it runs automatically when the signal is sent

D. To define a URL route in Django

Solution

1. Step 1: Understand what signals do in Django

   Signals notify parts of your app when something happens, like saving a model.

2. Step 2: Role of the @receiver decorator

   The decorator links a function to a signal so it runs automatically when the signal is sent.

3. Final Answer:

   To connect a function to a signal so it runs automatically when the signal is sent -> Option C

4. Quick Check:

   Receiver decorator connects functions to signals = D [OK]

Hint: Receiver links functions to signals for automatic execution [OK]

Common Mistakes:

• Confusing receiver with model or URL definitions
• Thinking receiver creates models or templates
• Mixing up signals with URL routing

2. Which of the following is the correct syntax to use the @receiver decorator for the post_save signal of a model named Book?

easy

A. @receiver(post_save, sender=Book)
def my_handler(sender, instance, **kwargs):
pass

B. @receiver(post_save(Book))
def my_handler(sender, instance, **kwargs):
pass

C. @receiver(sender=Book, post_save)
def my_handler(sender, instance, **kwargs):
pass

D. @receiver(post_save)
def my_handler(Book, instance, **kwargs):
pass

Solution

1. Step 1: Check the correct decorator syntax

   The @receiver decorator takes the signal as first argument and sender=ModelName as keyword argument.

2. Step 2: Match the correct function signature

   The handler function must accept sender, instance, and **kwargs.

3. Final Answer:

   @receiver(post_save, sender=Book) def my_handler(sender, instance, **kwargs): pass -> Option A

4. Quick Check:

   Correct decorator syntax = C [OK]

Hint: Use @receiver(signal, sender=Model) syntax [OK]

Common Mistakes:

• Passing sender as positional argument
• Incorrect function parameters
• Missing sender keyword argument

3. Given this code snippet, what will be printed when a Book instance is saved?

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

@receiver(post_save, sender=Book)
def notify(sender, instance, **kwargs):
    print(f"Book saved: {instance.title}")

book = Book(title='Django Basics')
book.save()

medium

A. Book saved: None

B. Error: receiver not connected

C. No output printed

D. Book saved: Django Basics

Solution

1. Step 1: Understand the signal connection

   The @receiver decorator connects notify to post_save for Book.

2. Step 2: What happens on book.save()?

   Saving the book triggers post_save, calling notify, which prints the book's title.

3. Final Answer:

   Book saved: Django Basics -> Option D

4. Quick Check:

   post_save triggers print with instance title = B [OK]

Hint: post_save calls receiver printing instance data [OK]

Common Mistakes:

• Assuming no output without explicit call
• Confusing instance attribute access
• Thinking receiver needs manual call

4. Identify the error in this signal receiver code:

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

@receiver(post_save, sender=Author)
def handle_save(instance, **kwargs):
    print(f"Author saved: {instance.name}")

medium

A. Incorrect decorator usage, should not use sender

B. Missing sender parameter in function definition

C. Function name must be post_save_handler

D. No error, code is correct

Solution

1. Step 1: Check function parameters for signal handlers

   Signal handlers must accept sender, instance, and **kwargs.

2. Step 2: Identify missing parameter

   The function handle_save lacks the sender parameter, causing an error.

3. Final Answer:

   Missing sender parameter in function definition -> Option B

4. Quick Check:

   Signal handler must have sender parameter = A [OK]

Hint: Signal handlers need sender, instance, **kwargs parameters [OK]

Common Mistakes:

• Omitting sender parameter
• Wrong function signature order
• Assuming function name matters

5. You want to run a function only when a User model instance is created (not updated). How do you use the @receiver decorator with post_save to achieve this?

hard

A. @receiver(post_save, sender=User)
def user_created(sender, instance, created, **kwargs):
if created:
print('New user created')

B. @receiver(post_save, sender=User)
def user_created(sender, instance, **kwargs):
print('New user created')

C. @receiver(post_save, sender=User)
def user_created(sender, instance, created=False, **kwargs):
if created == False:
print('New user created')

D. @receiver(post_save, sender=User)
def user_created(sender, instance, **kwargs):
if not created:
print('New user created')

Solution

1. Step 1: Understand the created flag in post_save

   The post_save signal passes a created boolean indicating if the instance was newly created.

2. Step 2: Use created to run code only on creation

   Check if created: inside the receiver function to run code only when a new instance is created.

3. Final Answer:

   @receiver(post_save, sender=User) def user_created(sender, instance, created, **kwargs): if created: print('New user created') -> Option A

4. Quick Check:

   Use created flag in post_save receiver = A [OK]

Hint: Check 'created' flag in post_save receiver to detect new instances [OK]

Common Mistakes:

• Ignoring the created parameter
• Checking created incorrectly
• Missing created parameter in function