Code 2

...class Presence:
    def handle_LOGIN(self, username):
        if self.state != 'needlogin':
            self.sendLine('ERROR')
            return
        if self.factory.userOnline(username):
            self.sendLine('NO')
        else:
            self.factory.addUser(username)
            self.username = username
            self.sendLine('YES')
            self.state = 'loggedin'