Question

I’m trying to sub-class db.Property and override the set method to implement stuff like pre and post set logic.

The problem is that __set__ is being called directly on the property by db.Model.__init__() during the from_entity conversion of entity to instance (after it comes out of the datastore), so obviously pre and post set logic should not be called.

class MyProperty(db.StringProperty):
    def __set__(model_instance, value):
        self.pre_set(value)
        super(MyProperty.__set__(model_instance, value)
        self.post_set(value)


class MyModel(db.Model):
    foo = MyProperty()

my_model = MyModel()
my_model.put()
my_model.foo = u'A new string.' """pre/post set logic runs."""

#onload the __set__ method will be called again
loaded_model = db.get(my_model.key())

# In db.Model.__init__()
for prop in self.properties().values():
    value = kwargs.get(prop.name, None) or prop.default() #or something like that
    prop.__set__(self, value) """pre/post set logic also runs :("""

How can I differentiate between these two occurrences without having to override db.Model.__init__()? or should I just do that? Am I not supposed to be doing this with prop.__set__()?

Answer 1

Unfortunately, there’s no easy way to distinguish the two situations. You could examine the stack, but that’s an extremely kludgy approach. You could override Model.__init__ – either by monkeypatching it or by requiring users of your property extend your custom model subclass – but I’m not sure how you’d modify it in a way that would help yet remain backwards compatible with existing property classes.

You might want to check out Guido’s NDB project – it may be more flexible in this respect, and is still under active development.