Inheriting assigned properties from a mix-in

I would like to use a mix-in class to add properties to a model.

from google.appengine.ext import db

class Taggable(object):
    tag_list = db.StringListPro开发者_JS百科perty()

    def attach_tag(self, tag):
        self.tag_list.append(tag)
        self.put()

    def remove_tag(self, tag):
        self.tag_list.pop(self.tag_list.index(tag))
        self.put()

class Post(db.Model, Taggable):
    title = db.TextProperty()

This is just an example, no need to chew my ear off about bad practices or something.

Currently, I have something similar to this, except I have to put tag_list = db.StringListProperty() outside of the mix-in (errors otherwise), this is messy code, and I would like to avoid that.

Simply, how can I assign properties (such as tag_list) to a model from within a mix-in?

Should not Taggable be based on db.Model instead of object?