无法使用pafy下载特定质量的文件

Question

playlist = pafy.get_playlist(self.params["url"])
videos = playlist['items']
quality = self.params['quality']
for video in videos :
    p = video['pafy']
    selectedVideo = p.quality
    selectedVideo.download()

但我有一个错误：

AttributeError:'YtdlPafy' object has no attribute 'quality'