code: def scan(self, paths=None, depth=2):
          
          song_exts = ['mp3', 'ogg', 'wma', 'm4a']
          exts = song_exts
          paths = paths or [Library.DEFAULT_MUSIC_FOLDER]
          depth = depth if depth <= 3 else 3
          media_files = []
          for directory in paths:
              logger.debug('正在扫描目录(%s)...', directory)
              media_files.extend(scan_directory(directory, exts, depth))
          logger.info('共扫描到 %d 个音乐文件，准备将其录入本地音乐库', len(media_files))
  
          for fpath in media_files:
              add_song(fpath, self._songs, self._artists, self._albums)
          logger.info('录入本地音乐库完毕')
docs: scan media files in all paths