Where should signal handlers live in a django project?

Question

I have just started implementing signal listeners in a django project. While I understand what they are and how to use them. I am having a hard time figuring out where I should put them. The documentation from the django site has this to say:

Where should this code live?

You can put signal handling and registration code anywhere you like. However, you'll need to make sure that the module it's in gets imported early on so that the signal handling gets registered before any signals need to be sent. This makes your app's models.py a good place to put registration of signal handlers.

While its a good suggestion, having non model classes or methods in my models.py just rubs me the wrong way.

So then, what is the best practice/rule for storing and registering signal handlers?

Answer 1

This was added to the documentation when Django 1.7 was released:

Strictly speaking, signal handling and registration code can live anywhere you like, although it’s recommended to avoid the application’s root module and its models module to minimize side-effects of importing code.

In practice, signal handlers are usually defined in a signals submodule of the application they relate to. Signal receivers are connected in the ready() method of your application configuration class. If you’re using the receiver() decorator, simply import the signals submodule inside ready().

Changed in Django 1.7: Since ready() didn’t exist in previous versions of Django, signal registration usually happened in the models module.

Best practice is to define your handlers in handlers.py in a signals submodule, e.g. a file that looks like:

yourapp/signals/handlers.py:

from django.db.models.signals import pre_save
from django.dispatch import receiver
from myapp.models import MyModel

@receiver(pre_save, sender=MyModel)
def my_handler(sender, **kwargs):
    pass

The best place to register your signal handler is then in the AppConfig of the app that defines it, using the ready() method. This will look like this:

yourapp/apps.py:

from django.apps import AppConfig

class TasksConfig(AppConfig):
    name = 'tasks'
    verbose_name = "Tasks"

    def ready(self):
        import yourproject.yourapp.signals.handlers #noqa

Make sure you're loading your AppConfig by specifying it either directly in your settings.py's INSTALLED_APPS, or in the __init__ of your app. See see the ready() documentation for more information.

Note: If you're providing signals for other apps to listen too as well, put them in the __init__ in your signals module, e.g. a file that looks like:

yourapp/signals/_init_.py

import django.dispatch

task_generate_pre_save = django.dispatch.Signal(providing_args=["task"])

Another app can then listen to your signal by importing and registering it, e.g. from yourapp.signals import task_generate_pre_save. Separating your signals from your handlers keeps things clean.

Instructions for Django 1.6:

If you're still stuck on Django 1.6 or lower, then you'd do the same thing (define your handlers in yourapp/signals/handlers.py) but rather than using AppConfig, you would load the handlers via the _init_.py of your app, e.g. something like:

yourapp/_init_.py

import signals

This isn't as nice as using the ready() method because it often causes circular import issues.

Answer 2

I actually like to make them classmethods of the model itself. That keeps everything within one class, and means you don't have to worry about importing anything.

Answer 3

I've only just come across this, and as my signals are not model-related I thought I'd add my solution.

I am logging various data around log in / log out, and needed to hook into django.contrib.auth.signals.

I have put the signal handlers into a signals.py file, and then imported signals from the __init__.py module file, as I believe this is called as soon as the app starts up (testing with a print statement suggests that it's called even before the settings file is read.)

# /project/__init__.py
import signals

and in signals.py

# /project/signals.py
from django.contrib.auth.signals import user_logged_in

def on_logged_in(sender, user, request, **kwargs):
    print 'User logged in as: \'{0}\''.format(user)

user_logged_in.connect(on_logged_in)

I'm pretty new to Django (/python) so am open to anyone telling me that this is a terrible idea!

Answer 4

I just recently read this article about best practices when it comes to lay out your projects/applications, and it suggests that all your custom dispatcher signals should go in a file called signals.py. However, that doesn't fully solve your problem, since you still need to import these somewhere, and the earlier they get imported the better.

The model suggestion is a good one. Since you already defined everything in your signals.py file, it shouldn't take more than a line at the top of the the file. This is similar to the way the admin.py file is laid out (with class definitions at the top and the code for registering all the custom admin classes at the bottom), if you define your signals then connect them in the same file.

Hope that helps! Ultimately it comes down to what you prefer.

Answer 5

models.py and signals.py in each app have been the recommended places to connect signals, however, they are not the best solution, in my opinion, to keep signals and handlers dispatched. Dispatching should be the reason signals and handlers invented in django.

I was struggling for long time, and finally we figured out the solution.

create a connector module in app folder

so we have:

app/
    __init__.py
    signals.py
    models.py
    connectors.py

in app/connectors.py, we defined signal handlers and connect them. An example is provided:

from signals import example_signal
from models import ExampleModel
from django.db.models.signals import post_save, post_delete

def hanndler(sender, *args, **kwargs):
    pass

post_save.connect(hander, sender=ExampleModel)

then in models.py, we add the following line in the end of the file:

from app import connector

Everything done here.

In this way, we can put signals in signals.py, and all the handlers in connectors.py. No mess in models and signals.

Hope it provides another solution.

Answer 6

Small reminder about AppConfig. Don't forget to set:

# yourapp/__init__.py

default_app_config = 'yourapp.apps.RockNRollConfig'

Answer 7

I keep them in a separate file signals.py , In models.py after all models are defined. I import them and connect models to signals.

signals.py

#  necessary imports

def send_mail_on_save(<args>):
    # code here 

models.py

# imports
class mymodel(models.Model):
    # model here

# import signals
from signals import send_mail_on_save
# connect them 
post_save.connect(send_mail_on_save,sender=mymodel)

This provides me logical separation, of course there is nothing wrong on keeping them in models.py , But it is more manageable this way.

Hope this helps!!