Using AuthSub on App Engine

For a full explanation, see this article: Retrieving Authenticated Google Data Feeds with Google App Engine

    # Initialize a client to talk to Google Data API services.
    client = gdata.service.GDataService()
    gdata.alt.appengine.run_on_appengine(client)

    session_token = None
    # Find the AuthSub token and upgrade it to a session token.
    auth_token = gdata.auth.extract_auth_sub_token_from_url(self.request.uri)
    if auth_token:
      # Upgrade the single-use AuthSub token to a multi-use session token.
      session_token = client.upgrade_to_session_token(auth_token)
    if session_token and users.get_current_user():
      # If there is a current user, store the token in the datastore and
      # associate it with the current user. Since we told the client to
      # run_on_appengine, the add_token call will automatically store the
      # session token if there is a current_user.
      client.token_store.add_token(session_token)
    elif session_token:
      # Since there is no current user, we will put the session token
      # in a property of the client. We will not store the token in the
      # datastore, since we wouldn't know which user it belongs to.
      # Since a new client object is created with each get call, we don't
      # need to worry about the anonymous token being used by other users.
      client.current_token = session_token