Google App Engine: Is it safe to reveal a user's user_id publicly

Question

Is there any personal identifying information linked to this number?

Are user_id's specific to my app or does a user have the same user_id across all app engine apps?

Answer 1

user_id is basically same across all GAE apps, and it might be possible to get other user info using user = User(_user_id = 'user_id'), so I'd recommend to avoid using user id's in URLs and other client-visible information.