django built-in signals 

Django's built-in Signals:
Django provides a set of built-in signals that let user code get notified by Django itself of certain actions. These include some useful notifications:

django.db.models.signals.pre_save & django.db.models.signals.post_save : Sent before or after a model's save() method is called
django.db.models.signals.pre_delete & django.db.models.signals.post_delete : Sent before or after a model's delete() method or queryset's delete() method is called
django.db.models.signals.m2m_changed : Sent when a ManyToManyField on a model is changed
django.core.signals.request_started & django.core.signals.request_finished : Sent when Django starts or finishes an HTTP request