I\'m starting to write an app whereby a mobile app (Android/IPhone) will communicate with the GAE backend (Python) through a series of Web API calls using JSON.
I can\'t
I cannot see why you would need a session? Sessions on App Engine are persisted in the data store, so if you can keep your requests stateless, I encourage you to do so.
As you will have your own user service which will authenticate the users, I suggest you use Digest authentication, as the secret is never included in the request.
There are libraries implementing Digest for most client and server platforms.