[Django]-How can I have a Django signal call a model method?

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:

1. 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.
   

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