I'm getting back to programming for Google App Engine and I've found, in old, unused code, instances in which I wrote constructors for models. It seems like a good idea, but there's no mention of it online and I can't test to see if it works. Here's a contrived example, with no error-checking, etc.:

class Dog(db.Model):
    name = db.StringProperty(required=True)
    breeds = db.StringListProperty()
    age = db.IntegerProperty(default=0)

    def __init__(self, name, breed_list, **kwargs):
        db.Model.__init__(**kwargs)
        self.name = name
        self.breeds = breed_list.split()

rufus = Dog('Rufus', 'spaniel terrier labrador')
rufus.put()

The **kwargs are passed on to the Model constructor in case the model is constructed with a specified parent or key_name, or in case other properties (like age) are specified. This constructor differs from the default in that it requires that a name and breed_list be specified (although it can't ensure that they're strings), and it parses breed_list in a way that the default constructor could not.

Is this a legitimate form of instantiation, or should I just use functions or static/class methods? And if it works, why aren't custom constructors used more often?