code: def next_song(self):
          
          # 如果没有正在播放的歌曲，找列表里面第一首能播放的
          if self.current_song is None:
              return self._get_good_song()
  
          if self.playback_mode == PlaybackMode.random:
              next_song = self._get_good_song(random_=True)
          else:
              current_index = self._songs.index(self.current_song)
              if current_index == len(self._songs) - 1:
                  if self.playback_mode in (PlaybackMode.loop, PlaybackMode.one_loop):
                      next_song = self._get_good_song()
                  elif self.playback_mode == PlaybackMode.sequential:
                      next_song = None
              else:
                  next_song = self._get_good_song(base=current_index+1)
          return next_song
docs: next song for player, calculated based on playback_mode