Receiver decorator in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a signal is sent in Django using a receiver?

Consider the following Django signal receiver code. What will be printed when the signal is sent?

Django

from django.dispatch import receiver, Signal

my_signal = Signal()

@receiver(my_signal)
def my_receiver(sender, **kwargs):
    print(f"Signal received from {sender}")

my_signal.send(sender='TestSender')

A TypeError due to missing arguments

B Signal received from <class 'str'>

C No output, signal not connected

D Signal received from TestSender

💡 Hint

Remember that the sender argument is passed as is to the receiver.

The receiver function prints the sender argument passed during signal sending. Here, sender='TestSender' is a string, so it prints exactly that.

📝 Syntax

intermediate

Which option correctly uses the @receiver decorator for a built-in Django signal?

You want to connect a function to Django's post_save signal for a model named MyModel. Which code snippet is correct?

Django

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

# Choose the correct receiver function below:

A

@receiver(post_save, sender=MyModel)
def my_handler(sender, instance, **kwargs):
    print('Saved')

B

@receiver(post_save)
def my_handler(sender, instance, **kwargs):
    print('Saved')

C

@receiver(post_save, sender=MyModel())
def my_handler(sender, instance, **kwargs):
    print('Saved')

D

@receiver(post_save, sender='MyModel')
def my_handler(sender, instance, **kwargs):
    print('Saved')

💡 Hint

The sender argument must be the model class, not a string or instance.

The sender parameter expects the model class itself, not a string or an instance. Option A correctly passes MyModel class.

🔧 Debug

advanced

Why does this receiver function never get called?

Given this code, the receiver function does not execute when the signal is sent. What is the cause?

Django

from django.dispatch import receiver, Signal

my_signal = Signal()

@receiver(my_signal)
def my_receiver(sender, **kwargs):
    print('Signal received')

# Signal sent before receiver is imported
my_signal.send(sender=None)

A The sender argument must not be None

B The signal object is not instantiated properly

C The signal is sent before the receiver function is imported and connected

D The receiver function has incorrect parameters

💡 Hint

Think about when the receiver function is registered to the signal.

If the signal is sent before the receiver is imported, the receiver is not connected yet and won't be called.

❓ state_output

advanced

What is the output count of signal calls with multiple receivers?

If two receiver functions are connected to the same signal, and the signal is sent once, how many times will the receivers be called?

Django

from django.dispatch import receiver, Signal

my_signal = Signal()

@receiver(my_signal)
def receiver_one(sender, **kwargs):
    print('Receiver one called')

@receiver(my_signal)
def receiver_two(sender, **kwargs):
    print('Receiver two called')

my_signal.send(sender=None)

A Only the first receiver is called

B Both receivers are called once, so two print statements appear

C Only the last receiver connected is called

D No receivers are called because of signal conflict

💡 Hint

Signals can have multiple receivers connected.

Django signals call all connected receivers when sent. Here, both print statements run once.

🧠 Conceptual

expert

What error occurs if a receiver function has a wrong signature?

Consider a receiver function connected to a Django signal but defined without accepting **kwargs. What happens when the signal is sent?

Django

from django.dispatch import receiver, Signal

my_signal = Signal()

@receiver(my_signal)
def my_receiver(sender):
    print('Signal received')

my_signal.send(sender=None, extra='data')

A TypeError: my_receiver() got an unexpected keyword argument 'extra'

B Signal received is printed without error

C AttributeError due to missing kwargs

D No output, receiver silently ignored

💡 Hint

Signal sends extra keyword arguments; receiver must accept them.

If the receiver does not accept **kwargs, extra keyword arguments cause a TypeError.

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