def __init__(self, service_name, user_name):
super(Storage, self).__init__(lock=threading.Lock())<EOL>self._service_name = service_name<EOL>self._user_name = user_name<EOL>
Constructor.

        Args:
            service_name: string, The name of the service under which the
                          credentials are stored.
            user_name: string, The name of the user to store credentials for.