pre_delete and post_delete signals in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Django model instance is deleted?

Consider a Django model with connected pre_delete and post_delete signals. What is the correct order of signal execution when an instance is deleted?

Django

from django.db.models.signals import pre_delete, post_delete
from django.dispatch import receiver
from myapp.models import MyModel

@receiver(pre_delete, sender=MyModel)
def before_delete(sender, instance, **kwargs):
    print('Before delete')

@receiver(post_delete, sender=MyModel)
def after_delete(sender, instance, **kwargs):
    print('After delete')

obj = MyModel.objects.create(name='test')
obj.delete()

A 2,1,3

B 3,2,1

C 1,2,3

D 1,3,2

💡 Hint

Think about what happens before and after the actual deletion.

The pre_delete signal runs before the model instance is deleted from the database. Then the instance is deleted. Finally, the post_delete signal runs after deletion.

❓ state_output

intermediate

What is printed by the signal handlers on deleting a model instance?

Given the following signal handlers connected to a Django model, what will be printed when an instance is deleted?

Django

from django.db.models.signals import pre_delete, post_delete
from django.dispatch import receiver
from myapp.models import MyModel

@receiver(pre_delete, sender=MyModel)
def before_delete(sender, instance, **kwargs):
    print(f'Preparing to delete {instance.name}')

@receiver(post_delete, sender=MyModel)
def after_delete(sender, instance, **kwargs):
    print(f'Deleted {instance.name}')

obj = MyModel.objects.create(name='Sample')
obj.delete()

A

Preparing to delete Sample
Deleted Sample

B

Deleted Sample
Preparing to delete Sample

C Preparing to delete Sample

D Deleted Sample

💡 Hint

Remember the order of pre_delete and post_delete signals.

The pre_delete handler prints before deletion, and the post_delete handler prints after deletion. Both messages appear in that order.

📝 Syntax

advanced

Identify the syntax error in connecting a pre_delete signal

Which option contains a syntax error when connecting a pre_delete signal to a Django model?

A

@receiver(pre_delete, sender=MyModel)
def handler(sender, instance, **kwargs):
    pass

B

@receiver(pre_delete sender=MyModel)
def handler(sender, instance, **kwargs):
    pass

C pre_delete.connect(handler, sender=MyModel)

D pre_delete.connect(handler sender=MyModel)

💡 Hint

Look carefully at the decorator syntax.

Option B is missing a comma between pre_delete and sender=MyModel in the decorator, causing a syntax error.

🔧 Debug

advanced

Why does the post_delete signal not run after deleting a model instance?

A developer notices that the post_delete signal handler is never called after deleting a model instance. Which of the following is the most likely cause?

A The signal handler was connected with the wrong sender or not connected at all.

B The <code>pre_delete</code> signal handler raises an exception preventing deletion.

C The model's delete() method was overridden but did not call super().delete().

D The model instance was deleted using a queryset's delete() method, which bypasses signals.

💡 Hint

Check how the signal handler is connected and the sender argument.

If the signal handler is not connected properly or the sender is incorrect, the post_delete signal will not be triggered.

🧠 Conceptual

expert

What is the main difference between pre_delete and post_delete signals in Django?

Choose the option that best describes the key conceptual difference between pre_delete and post_delete signals in Django.

A <code>pre_delete</code> runs only if the instance has related objects; <code>post_delete</code> runs otherwise.

B <code>pre_delete</code> runs after the instance is removed; <code>post_delete</code> runs before deletion starts.

C <code>pre_delete</code> only runs for bulk deletes; <code>post_delete</code> runs for single instance deletes.

D <code>pre_delete</code> runs before the database deletion; <code>post_delete</code> runs after the instance is removed from the database.

💡 Hint

Think about when each signal is triggered relative to the actual deletion.

pre_delete is triggered just before the model instance is deleted from the database. post_delete is triggered immediately after the deletion.