How can I have a Django signal call a model method?

Question

Maybe it's just late, but I cannot figure out why this isn't working. When I have a post_save signal call a generic function, it works, but when I have a post_save signal call a method from a model, nothing happens. Here is code that works:

class Revision(models.Model):
    # Model junk...

def send_email(sender, instance, created, **kwargs):
    if created:
        print "DO STUFF"

signals.post_save.connect(send_email, sender=Revision)

But this does not work:

class Revision(models.Model):
    # Model junk...

    def send_email(sender, instance, created, **kwargs):
        if created:
            print "DO STUFF"

signals.post_save.connect(Revision.send_email, sender=Revision)

Is there a kind soul out there who will keep me from smashing my head into the wall? Thanks.

Maybe helpful? [Django pre_save signal does not work](http://stackoverflow.com/q/6153730/143804) — ire_and_curses, Sep 23 '12 at 06:16
The best practice is to put all your signals in a `signals.py`; don't intermingle them with the django model class. — Burhan Khalid, Sep 23 '12 at 08:47

score 33 · Accepted Answer · edited May 23 '17 at 12:24

It seems to me that the problem in the second one is you are using an unbounded method send_mail. If you really want to call send_mail from within a class, maybe @classmethod or @staticmethod will help you out:

class Revision(models.Model):
    # Model junk...

    @classmethod
    def send_email(cls, sender, instance, created, **kwargs):
        if created:
            print "DO STUFF"

signals.post_save.connect(Revision.send_email, sender=Revision)

or

class Revision(models.Model):
    # Model junk...

    @staticmethod
    def send_email(sender, instance, created, **kwargs):
        if created:
            print "DO STUFF"

signals.post_save.connect(Revision.send_email, sender=Revision)

Alternatively without using these decorators, you can pass the bounded instance method:

class Revision(models.Model):
# Model junk...

    def send_email(self, sender, instance, created, **kwargs):
        if created:
            print "DO STUFF"

signals.post_save.connect(Revision().send_email, sender=Revision)

References:

From the Django source code:

def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
    """
    Connect receiver to sender for signal.

    Arguments:

        receiver
            A function or an instance method which is to receive signals.
            Receivers must be hashable objects.

            If weak is True, then receiver must be weak-referencable (more
            precisely saferef.safeRef() must be able to create a reference
            to the receiver).

            Receivers must be able to accept keyword arguments.

            If receivers have a dispatch_uid attribute, the receiver will
            not be added if another receiver already exists with that
            dispatch_uid.

Difference between @classmethod and @staticmethod: What is the difference between @staticmethod and @classmethod in Python?

Your third example using the instance method, the instance being called here is a new one, right, not the instance that was —here— saved? Edit: Oh but the `instance` variable *is* the one being saved. Seems a bit silly to do it like that, but I see that you're just showing you *can* do that if you need to. Cool. — Oli, Sep 23 '16 at 12:01
I don't think that third example works. From my testing, instance methods are not the same across all instances — smac89, Aug 04 '21 at 18:03

How can I have a Django signal call a model method?

1 Answers1

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