We are going to look at how Django signals work in this tutorial. Signals allow certain senders to notify a set of receivers that some action has taken place.

We will focus on the post_save signal in this tutorial, you can checkout a list of all signals here list of all signals

EXAMPLE

Assume you have a simple app to record book name and pages, a model for that app might look as shown below

class Book(models.Model):
    name = models.CharField(max_length=50)
    pages = models.CharField(max_length=50)

    def __str__(self):
        return self.name

Now assume you want to do something once a new book entry has been added to the database, how do you do that ? this is where signals come in.

HOW TO USE SIGNALS

Create a new file named signals.py in our app on the same level as models.py file.

mysite/signals.py

from django.db.models.signals import post_save
from django.dispatch import receiver

from mysite.models import Book


@receiver(post_save, sender=Book)
def Notify_if_book_is_saved(sender, **kwargs):
    if kwargs['created']:
        print("New book saved.")

The Notify_if_book_is_saved is our receiver function which will only be called when an instance of Book is saved, It simply prints " New book saved. " once a new book is saved.

Notice that our function takes a sender argument and keyword arguments (**kwargs); all signal handlers must take these arguments.

if keyword arguments (**kwargs) is new to you checkout my tutorial on that what are python args and kwargs

We can check whether an instance of Book was saved using if kwargs['created']:

created is a boolean sent to the receiver function as a keyword argument by post_save signal, checkout other arguments that are sent by post_save signal here Arguments sent with post_save signal

Modify your app.py as follows

mysite/app.py

from django.apps import AppConfig

class MysiteConfig(AppConfig):
    name = 'mysite'
    verbose_name = "Mysite"

    def ready(self):
        import mysite.signals
        pass

The next step will depend of the version of Django you are using.

if you are using Django 1.8+ , the documentation says that

New applications should avoid default_app_config. Instead they should require the dotted path to the appropriate AppConfig subclass to be configured explicitly in INSTALLED_APPS.

Example:

INSTALLED_APPS = [
   'mysite.apps.MysiteConfig',
]

For older versions of Django set the default_app_config variable as shown below inside __init__ .py file on same level as models.py .

mysite/__init__ .py

default_app_config = 'mysite.apps.MysiteConfig'

GET SOURCE CODE

https://github.com/felix13/django-signals

RESOURCES

Signals

header photo by Alex Knight on unsplash