Receiver decorator in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @receiver decorator in Django?

The @receiver decorator connects a function to a Django signal, so the function runs automatically when that signal is sent.

beginner

How do you import the <code>@receiver</code> decorator in Django?

You import it with: <code>from django.dispatch import receiver</code>.

intermediate

Explain the role of the signal argument in the @receiver(signal) decorator.

The signal argument tells Django which signal the decorated function should listen to and respond when that signal is sent.

intermediate

What parameters does a receiver function typically accept?

A receiver function usually accepts sender, instance, and **kwargs to get details about the signal event.

intermediate

Why is using the @receiver decorator better than manually connecting signals with signal.connect()?

The @receiver decorator makes the code cleaner and easier to read by linking the function directly to the signal in one place.

What does the @receiver decorator do in Django?

A Runs a function immediately

B Creates a new signal

C Deletes a signal

D Connects a function to a signal

The @receiver decorator connects a function to a Django signal so it runs when the signal is sent.

Which module do you import @receiver from?

A django.models

B django.signals

C django.dispatch

D django.utils

You import @receiver from django.dispatch.

What argument do you pass to the @receiver decorator?

A The signal to listen for

B The sender class

C The function name

D The model name

You pass the signal you want the function to listen for to the @receiver decorator.

Which of these is NOT a typical parameter of a receiver function?

A request

B instance

C sender

D **kwargs

Receiver functions usually accept sender, instance, and **kwargs, but not request.

Why use the @receiver decorator instead of signal.connect()?

A It runs the function faster

B It makes code cleaner and easier to read

C It disables the signal

D It automatically deletes the function

The @receiver decorator links the function to the signal clearly in one place, improving code readability.

Describe how the @receiver decorator works in Django and why it is useful.

Think about how Django signals notify parts of your app and how the decorator helps handle that.

Explain the typical parameters a receiver function accepts and what information they provide.

Consider what details the function needs to respond properly to the signal.

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