How to set your autoincrement primary key in AppEngine/Python?

This has already been implemented. In addition to BigTable distributes keys and a numeric id. DB query by id:
\rModel.get_by_id()
To get the id of any entity — entity.key().id()
\r
It is quite suitable for a url, only to note that id will incremented for entities of any type in your application. One at all. Therefore, the numbers are big. For example, www.arhimir.ru/news/1126006 — done on GAE-Python.

probably have to use additional model:
\r
\r

class Sequence(db.Model): Key: db.StringProperty() #key, for example "ArticleId" Number: db.IntegerProperty(default=1) #the value itself 

and the meaning is — that in one transaction, first get the Number value and then update it by +1