pre_delete and post_delete signals in Django - Step-by-Step Execution

Concept Flow - pre_delete and post_delete signals

Delete request triggered

↓

pre_delete signal sent

↓

Object deletion from DB

↓

post_delete signal sent

↓

Cleanup or follow-up actions

When deleting an object, Django first sends a pre_delete signal, then deletes the object, and finally sends a post_delete signal for any cleanup.

Execution Sample

Django

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

@receiver(pre_delete, sender=MyModel)
def before_delete(sender, instance, **kwargs):
    print(f"About to delete: {instance}")

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

This code prints messages before and after deleting a MyModel instance.

Execution Table

Step	Signal/Event	Action	Instance State	Output
1	Delete request	User calls delete()	Instance exists in DB	No output
2	pre_delete	Signal sent before deletion	Instance still exists	Prints: About to delete: <instance>
3	Delete object	Object removed from DB	Instance removed	No output
4	post_delete	Signal sent after deletion	Instance no longer in DB	Prints: Deleted: <instance>
5	End	No more actions	Instance deleted	Process complete

💡 Deletion process ends after post_delete signal is sent

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
instance	Exists in DB	Exists in DB	Deleted from DB	Deleted from DB	Deleted from DB

Key Moments - 3 Insights

Why does pre_delete signal see the instance still in the database?

Because pre_delete is sent before the actual deletion (see execution_table step 2), the instance still exists in the database at that moment.

Can post_delete signal access the instance data?

Yes, post_delete receives the instance data but the object is already deleted from the database (execution_table step 4). The instance is a copy passed to the signal.

What happens if deletion is canceled during pre_delete?

If an error or exception occurs in pre_delete handlers, the deletion can be stopped before step 3, so post_delete won't run.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the instance at step 3?

A Instance still exists in the database

B Instance has been deleted from the database

C Instance is being deleted but still accessible

D Instance is recreated

💡 Hint

Check the 'Instance State' column at step 3 in the execution_table

At which step does the pre_delete signal run?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Signal/Event' column in the execution_table

If the pre_delete signal raises an error, what happens next?

A post_delete runs but deletion is rolled back

B Deletion continues and post_delete runs

C Deletion stops and post_delete does not run

D Nothing happens, deletion proceeds normally

💡 Hint

Refer to key_moments about cancellation during pre_delete

Concept Snapshot

Django deletion flow:
- pre_delete signal fires before object is deleted
- Object is deleted from database
- post_delete signal fires after deletion
Use pre_delete to prepare or cancel deletion
Use post_delete for cleanup after deletion

Full Transcript

When you delete an object in Django, the system first sends a pre_delete signal. This happens before the object is removed from the database, so the instance still exists and can be accessed. After that, Django deletes the object from the database. Then, it sends a post_delete signal, which happens after the object is gone. The post_delete signal still receives the instance data but the object no longer exists in the database. If an error happens during pre_delete, the deletion can stop and post_delete will not run. This lets you prepare or cancel deletions and clean up after deletions safely.