Why is post_save being raised twice during the save of a Django model?

Question

I am attaching a method to the post_save signal of my Django model. This way I can clear some cached items whenever the model is modified.

The problem I am having is that the signal is being triggered twice when the model is saved. It doesn't necessarily hurt anything (the code will just gracefully error out) but it can't be right.

A quick example, just printing the model to the console (using the dev server):

from blog.models import Post
from django.db.models import signals

def purge_cache(sender, **kwargs):
    print 'Purging %s' % sender

signals.post_save.connect(purge_cache, sender=Post)

This is using the stable 1.1.1 release of Django.

Updated Information:

With feedback from everyone's comments, I have modified my question because the issue is now discovering why the post_save is being triggered twice. My guess at the moment is that my models.py code is imported twice and that the post_save is getting connected multiple times.

What would be the best way to figure out why it is being imported/ran twice?

Answer 1

While looking for the root of this problem, you can use quick workaround to prevent registering signal twice:

signals.post_save.connect(my_handler, MyModel, dispatch_uid="path.to.this.module")

Source.

Answer 2

Here is the ticket about this issue: Django's signal framework may register listeners more than once #3951. It is now fixed in SVN version of Django.

The problem is exactly as You said: Your module which registers signal, is loaded couple of times, in some cases by different import paths, thus each imported modules this way are wrongly interpreted by Django as different modules which registers the same signal.

Answer 3

Apparently, Python is sensitive to the way you import modules. In my case, it wasn't an issue with any of import code inside my blog application but an issue with the INSTALLED_APPS configuration, which I assume is used by Django to do an initial import.

Inside my blog application I was using imports such as:

from blog.models import *

My settings.py was configured as:

INSTALLED_APPS = (
    'django.contrib.admin',
    'django.contrib.auth',
    ...snip...
    'sorl.thumbnail',
    'mysite.blog',
)

The "mysite" prefix was added because I originally had import path issues when deploying the site. Later I fixed this issue (so it acted the same as the development server) by adding multiple paths in my WSGI script.

Removing the "mysite" prefix from the settings.py fixed the issue:

INSTALLED_APPS = (
    'django.contrib.admin',
    'django.contrib.auth',
    ...snip...
    'sorl.thumbnail',
    'blog',
)