Receiver decorator in Django - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the receiver decorator from Django signals.

Django

from django.dispatch import [1]

Drag options to blanks, or click blank then click option'

Areceiver

Bsignal

Cconnect

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'signal' instead of 'receiver' for import.

Trying to import 'connect' which is a method, not a decorator.

The receiver decorator is imported from django.dispatch to connect functions to signals.

2fill in blank

medium

Complete the code to decorate the function so it listens to the post_save signal.

Django

@receiver([1])
def my_handler(sender, instance, **kwargs):
    pass

Drag options to blanks, or click blank then click option'

Apost_delete

Bpre_save

Cpost_save

Dpre_delete

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pre_save' which runs before saving.

Confusing delete signals with save signals.

The post_save signal triggers after a model instance is saved, so the function listens to it.

3fill in blank

hard

Fix the error in the decorator to correctly listen to the signal from the User model.

Django

@receiver(post_save, sender=[1])
def user_saved(sender, instance, **kwargs):
    print('User saved')

Drag options to blanks, or click blank then click option'

Aauth.User

Bmodels.User

Cuser

DUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string like 'auth.User' instead of the User class.

Using lowercase 'user' which is undefined.

The sender must be the User model class, usually imported as 'User', not a string or module path.

4fill in blank

hard

Fill both blanks to connect a function to the pre_delete signal for the Article model.

Django

@receiver([1], sender=[2])
def article_deleted(sender, instance, **kwargs):
    print('Article deleted')

Drag options to blanks, or click blank then click option'

Apre_delete

Bpost_save

CArticle

DUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using post_save signal which is unrelated to deletion.

Using User model instead of Article.

Use pre_delete signal and the Article model class as sender to listen before an article is deleted.

5fill in blank

hard

Fill all three blanks to create a receiver for the m2m_changed signal on the tags field of the Post model.

Django

@receiver([1], sender=[2].tags.through)
def tags_changed(sender, instance, action, **kwargs):
    if action == [3]:
        print('Tags updated')

Drag options to blanks, or click blank then click option'

Am2m_changed

BPost

C'post_add'

Dpost_save

Attempts:

3 left

💡 Hint

Common Mistakes

Using post_save signal which does not track m2m changes.

Using 'pre_add' instead of 'post_add' for action.

Using wrong sender model.

The m2m_changed signal listens to many-to-many changes; sender is Post.tags.through; action 'post_add' means tags were added.

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