This example is a snippet taken from the Extending Django User Profile like a Pro

from django.db import models
from django.contrib.auth.models import User
from django.db.models.signals import post_save
 
class UserProfile(models.Model):
    user = models.OneToOneField(User, related_name='user')
    website = models.URLField(default='', blank=True)
    bio = models.TextField(default='', blank=True)

def create_profile(sender, **kwargs):
    user = kwargs["instance"]
    if kwargs["created"]:
        user_profile = UserProfile(user=user)
        user_profile.save()
post_save.connect(create_profile, sender=User)

from django.db import models
from django.contrib.auth.models import User
from django.db.models.signals import post_save
from django.dispatch import receiver
 
class UserProfile(models.Model):
    user = models.OneToOneField(User, related_name='user')
    website = models.URLField(default='', blank=True)
    bio = models.TextField(default='', blank=True)

@receiver(post_save, sender=UserProfile)
def post_save_user(sender, **kwargs):
    user = kwargs.get('instance')
    if kwargs.get('created'):
        ...

By utilizing the pre_save we can determine if a save action on our database was about updating an existing object or creating a new one.

In order to achieve this you can check the state of the model object:

    @receiver(pre_save, sender=User)
    def pre_save_user(sender, instance, **kwargs): 
        if not instance._state.adding:
            print ('this is an update')
        else:
            print ('this is an insert')

Now every time a save action takes place, the pre_save signal will run and will print:

• this is an update if the action derived from an update action.
• this is an insert if the action derived from an insert action.

Note that this method does not require any additional database queries.

Django's signals are restricted to precise class signatures upon registration, and thus subclassed models are not immediately registered onto the same signal.

Take this model and signal for example

class Event(models.Model):
    user = models.ForeignKey(User)


class StatusChange(Event):
    ...


class Comment(Event):
    ...


def send_activity_notification(sender, instance: Event, raw: bool, **kwargs):
    """
    Fire a notification upon saving an event
    """

    if not raw:
        msg_factory = MessageFactory(instance.id)
        msg_factory.on_activity(str(instance))
post_save.connect(send_activity_notification, Event)

With extended models, you must manually attach the signal onto each subclass else they won't be effected.

post_save.connect(send_activity_notification, StatusChange)
post_save.connect(send_activity_notification, Comment)

With Python 3.6, you can leverage some additional class methods build into classes to automate this binding.

class Event(models.Model):

    @classmethod
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        post_save.connect(send_activity_notification, cls)