This code should automatically connect players when they enter a game.
But the problem is when two users try to connect at the same time - in this case 2nd user can easily overwrite changes made by 1st user ('room_1' variable).

How could I make it thread safe?

def join(userId):
    users = memcache.get('room_1')
    users.append(userId)
    memcache.set('room_1', users)
    return users

I'm using Google App Engine (python) and going to implement simple game-server for exchanging peers given by Adobe Stratus.