def watch_once(self, key, timeout=None, **kwargs):
        """
        Watch a key and stops after the first event.

        If the timeout was specified and event didn't arrived method
        will raise ``WatchTimedOut`` exception.

        :param key: key to watch
        :param timeout: (optional) timeout in seconds.
        :returns: ``Event``
        """
        event_queue = queue.Queue()

        def callback(event):
            event_queue.put(event)

        watch_id = self.add_watch_callback(key, callback, **kwargs)

        try:
            return event_queue.get(timeout=timeout)
        except queue.Empty:
            raise exceptions.WatchTimedOut()
        finally:
            self.cancel_watch(watch_id)
Watch a key and stops after the first event.

        If the timeout was specified and event didn't arrived method
        will raise ``WatchTimedOut`` exception.

        :param key: key to watch
        :param timeout: (optional) timeout in seconds.
        :returns: ``Event``