We are super excited about App Engine\'s support for Google Cloud Endpoints.
That said we don\'t use OAuth2 yet and usually authenticate users with username/password
I wrote a custom python authentication library called Authtopus that may be of interest to anyone looking for a solution to this problem: https://github.com/rggibson/Authtopus
Authtopus supports basic username and password registrations and logins, as well as social logins via Facebook or Google (more social providers could probably be added without too much hassle too). User accounts are merged according to verified email addresses, so if a user first registers by username and password, then later uses a social login, and the verified email addresses of the accounts match up, then no separate User account is created.