Signal Connection Decorators

[zvoase]

Usually, signal receivers are defined as functions and then connected to a specific signal via a function call to the signal instance's connect method. The problem is, this registration of the receiver is located outside the receiver's definition. This can cause clutter, it violates DRY, and it is not very Pythonic in style. Several examples of the current usage pattern are included in the ​signal docs, so I don't need to go into too much detail.

I propose a change to the connect method on the django.dispatch.dispatcher.Signal class, which would allow you to decorate a signal receiver function and therefore skip the explicit call to the method. The usage of the new method would look something like this:

from django.db.models.signals import pre_save # Just an example of a signal.

@pre_save.connect(sender=MyModel) # Other keyword arguments may be given.
def receiver(sender, instance, *args, **kwargs):
    pass # Do something here.

---

I've written a patch which does exactly this, preserving backwards compatibility also (although that's not the biggest issue right now; signals are a very recent addition). It works by moving the current Signal.connect method to Signal._connect, replacing it with a small wrapper around the original which, when called, does one of two things:

• If called with positional arguments (and, optionally, keyword arguments), it behaves like the old Signal.connect, connecting the given receiver function to the signal. This allows it to be used as both a decorator and a registration function.
• If called with keyword arguments but no positional arguments, it returns a wrapper function, allowing it to decorate the receiver function and include the given keyword arguments (thanks to Python's lexical scoping).

This means it will work like so (with the effects easy to determine from the code):

from django.db.models.signals import pre_save

# Decorator with no args.
@pre_save.connect
def recvr1(sender, instance, *args, **kwargs):
    pass # ...

# Registration with only the receiver.
pre_save.connect(recvr1)

# Decorator with keyword args.
@pre_save.connect(sender=MyModel)
def recvr2(sender, instance, *args, **kwargs):
    pass # ...

# Registration with both receiver and keyword args.
pre_save.connect(recvr2, sender=MyModel)

[zvoase]

Patch-v1.diff

[zvoase]

I've just realised that the patch is not from the top of the source tree. I'm on the verge of kicking myself.

Will submit a better patch promptly.

[zvoase]

Patch-v2.diff

[zvoase]

Submitted new patch, claimed the ticket, and made the executive decision not to move the 'connect' method around, in the interest of not (potentially) screwing things up.

[anonymous]

Milestone post-1.0 deleted

[bendavis78]

Marking as Needs tests. I'd love to see this get into 1.2.

[bendavis78]

Also marking as "Needs documentation", since this affects the public api

[anon]

wouldn't it be nicer to have the decorator like this?

from django.db.models.signals import connect, pre_save

# Decorator with no args.
@connect(pre_save)
def recvr1(sender, instance, *args, **kwargs):
    pass # ...

# Decorator with keyword args.
@connect(pre_save, sender=MyModel)
def recvr2(sender, instance, *args, **kwargs):
    pass # ...

[anonymous]

Untested example:

def connect(signal, **kwargs):
	def receive(receiver):
		signal.connect(receiver, **kwargs)
		return receiver
	return receive

[brosner]

I've made a slightly new patch and started discussion at ​http://groups.google.com/group/django-developers/msg/eb8cd203df82ad5a

[gsakkis]

I use a slightly more general version of the one posted on 10/04/09 that allows multiple signals at once:

def connect(*signals, **kwds):
    ...

@connect(post_save, post_delete, sender=MyModel)
def delete_stale_cache(sender, instance, **kwds):
    cache.delete('mymodel_%s' % instance.pk)

[jezdez]

Removed the needs tests and docs flags referring to brosner's patch.

[brosner]

Moving to accepted until I can get my patch in.

[brosner]

(In [13773]) Fixed #9015 -- added a signal decorator for simplifying signal connections

[anonymous]

Function decorators usually named in the form of a verb - cache_page, require_POST, etc. - so why is this decorator named receiver instead of connect or receive_signal?

[jezdez]

Replying to anonymous:

Function decorators usually named in the form of a verb - cache_page, require_POST, etc. - so why is this decorator named receiver instead of connect or receive_signal?

I have to say, that's a valid point, e.g. this looks nicer to me, too:

@receive_signal(post_login)
def update_last_seen(sender, user, **kwargs):
    user.last_seen = datetime.now()
    user.save()

[jezdez]

Enabling the connect to act as a decorator

[jezdez]

I've pushed a small update to my branch at ​https://github.com/jezdez/django/tree/feature%2Fsignal-decorator

[aaugustin]

1.3 was released with @receiver. It doesn't seem worth changing the API now that it has gone public.