Generating a unique data store key from a federated identity
User.federated_identity() "Returns the user's OpenID identifier.", which is unique by definition (it's a URL that uniquely identifies the user).
I need a unique datastore key for users authenticated via openid with the python 2.7 runtime for the google apps engine.  
Should I use User.federated_identity() or User.federated_provider() + User.federated_identity()?  
In other words is User.federated_identity() unique for ALL providers or just one specific provider?