Signal dispatch process in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Django signal is sent?

Consider a Django signal connected to multiple receivers. What is the order in which receivers are called when the signal is sent?

A Receivers are called randomly without any guaranteed order.

B Receivers are called in reverse order of connection.

C Receivers are called in alphabetical order of their function names.

D Receivers are called in the order they were connected to the signal.

💡 Hint

Think about how Django stores signal receivers internally.

Django calls signal receivers in the order they were connected. This means the first receiver connected is called first, then the next, and so on.

📝 Syntax

intermediate

Identify the correct way to connect a receiver to a Django signal

Which of the following code snippets correctly connects a receiver function to Django's post_save signal?

Django

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

# Receiver function
@receiver(post_save, sender=MyModel)
def my_handler(sender, instance, created, **kwargs):
    pass

A

post_save.connect(my_handler)
def my_handler(sender, instance, created, **kwargs): pass

B

post_save.connect(my_handler, sender=MyModel)
def my_handler(sender, instance, created, **kwargs): pass

C

@receiver(post_save, sender=MyModel)
def my_handler(sender, instance, created, **kwargs): pass

D

@receiver(post_save)
def my_handler(sender, instance, created, **kwargs): pass

💡 Hint

Look for the decorator that specifies the sender.

Using the @receiver decorator with both the signal and sender specified is the correct and clean way to connect a receiver.

🔧 Debug

advanced

Why does this Django signal receiver not get called?

Given the following code, why does the receiver function never execute when a MyModel instance 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=object)
def my_handler(sender, instance, created, **kwargs):
    print('Signal received')

# Somewhere else in the code
obj = MyModel()
obj.save()

A The receiver is not connected to the specific sender MyModel, so it won't be called.

B The signal post_save is not imported correctly.

C The print statement inside the receiver is invalid in Django signals.

D The receiver function must be connected manually using connect() method.

💡 Hint

Check if the receiver is listening to the right sender.

Without specifying sender=MyModel, the receiver listens to all models. But if the signal is sent with a sender filter, the receiver may not be called. Usually, specifying sender is needed to ensure the receiver is called for that model.

❓ state_output

advanced

What is the output when multiple receivers modify the same instance?

If two receivers connected to the post_save signal modify the same model instance's attribute, what will be the final value after saving?

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 receiver_one(sender, instance, **kwargs):
    instance.name = 'First'
    instance.save()

@receiver(post_save, sender=MyModel)
def receiver_two(sender, instance, **kwargs):
    instance.name = 'Second'
    instance.save()

obj = MyModel(name='Original')
obj.save()
print(MyModel.objects.get(pk=obj.pk).name)

A Raises a RuntimeError due to recursive save calls

B Second

C Original

D First

💡 Hint

Think about what happens when save() is called inside a post_save receiver.

Calling save() inside a post_save receiver triggers the signal again, causing infinite recursion and eventually a RuntimeError.

🧠 Conceptual

expert

How does Django ensure thread safety in signal dispatch?

Django signals can be used in multi-threaded environments. How does Django ensure that signal receivers are called safely without conflicts?

A Django uses a thread-safe internal lock to serialize signal dispatch calls.

B Django copies the list of receivers before dispatching to avoid modification during iteration.

C Django runs each receiver in a separate thread automatically.

D Django requires developers to manually handle thread safety in signal receivers.

💡 Hint

Consider how modifying a list during iteration can cause issues.

Django copies the list of connected receivers before calling them to avoid issues if receivers are added or removed during dispatch, ensuring thread safety.