code: def current_song(self, song):
          
          self._last_song = self.current_song
          if song is None:
              self._current_song = None
          # add it to playlist if song not in playlist
          elif song in self._songs:
              self._current_song = song
          else:
              self.insert(song)
              self._current_song = song
          self.song_changed.emit(song)
docs: 设置当前歌曲，将歌曲加入到播放列表，并发出 song_changed 信号
  
          .. note::
  
              该方法理论上只应该被 Player 对象调用。