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.