What are signals in Django, why are they needed, and what are the main signals used?

Signals in Django are a mechanism that allows application components to respond to certain events without tight coupling between them. This is an implementation of the "observer" pattern.

Why they are needed:

• They allow separating the logic that should be triggered by specific actions (e.g., creating a user, saving a model).
• They simplify extending functionality without changing the core code.

Main signals that are often used:

• pre_save and post_save — called before and after saving a model object.
• pre_delete and post_delete — before and after deleting an object.
• m2m_changed — when many-to-many relationships change.
• request_started and request_finished — at the start and end of an HTTP request.

Example of using post_save:

from django.db.models.signals import post_save
from django.dispatch import receiver
from django.contrib.auth.models import User

@receiver(post_save, sender=User)
def user_created(sender, instance, created, **kwargs):
    if created:
        print(f"A new user has been created: {instance.username}")

Thus, signals help organize responses to events in the application more flexibly and modularly.