code: def get_song_url(self, song_id, bit_rate=320000):
          
  
          url = 'http://music.163.com/weapi/song/enhance/player/url?csrf_token='
          csrf = ''
          params = {'ids': [song_id], 'br': bit_rate, 'csrf_token': csrf}
          result = self.post_request(url, params)
          song_url = result['data'][0]['url']  # download address
  
          if song_url is None:  # Taylor Swift's song is not available
              LOG.warning(
                  'Song %s is not available due to copyright issue. => %s',
                  song_id, result)
              raise SongNotAvailable(
                  'Song {} is not available due to copyright issue.'.format(song_id))
          else:
              return song_url
docs: Get a song's download address.
  
          :params song_id: song id<int>.
          :params bit_rate: {'MD 128k': 128000, 'HD 320k': 320000}
          :return: a song's download address.