What Are Signals?

Signals allow you to make Django call custom methods after common actions using a dispatcher. Beyond that, I really don't know much else. Please help expand this documentation, if you can.

The author of Zyons ​talks about signals a bit here.

Built-In Signals

Django has the following build in signalers:

• class_prepared
• pre_init
• post_init
• pre_save
• post_save
• pre_delete
• post_delete
• post_syncdb

All the above are related to classes in your models.py files.

Custom Signals

You can send custom signals from the dispatcher, as well. In the following example from ​this intro to signals, a signal called "object_viewed" is sent whenever the following bit is called:

dispatcher.send(signal=signals.object_viewed, request=request, object=object)

You'll have to instantiate the signal in a file called "signals.py" in your app's folder:

object_viewed=object()

And then, in any file in which you use that signal (typically, models.py and views.py), you'll need to import your signals file:

from project.app import signals

Example

The following is taken from ​Zyons, the only place I've seen signals used so far. Unless I'm completely incorrect, the following code has the dispatcher call "increment_tag_summary" before a TagUserObject object is saved, and "decrement_tag_summary" after a TagUserObject object is deleted.

models.py

from django.db.models import signals
from django.dispatch import dispatcher

[...]

def increment_tag_summary(sender, instance, signal, *args, **kwargs):
    [...]

def decrement_tag_summary(sender, instance, signal, *args, **kwargs):
    [...]

dispatcher.connect( increment_tag_summary , signal=signals.pre_save, sender=TagUserObject )
dispatcher.connect( decrement_tag_summary , signal=signals.post_delete, sender=TagUserObject )