code: def get_playlist_songs(self, playlist_id, limit=1000):
          
  
          url = 'http://music.163.com/weapi/v3/playlist/detail?csrf_token='
          csrf = ''
          params = {'id': playlist_id, 'offset': 0, 'total': True,
                    'limit': limit, 'n': 1000, 'csrf_token': csrf}
          result = self.post_request(url, params)
  
          songs = result['playlist']['tracks']
          songs = [Song(song['id'], song['name']) for song in songs]
          return songs
docs: Get a playlists's all songs.
  
          :params playlist_id: playlist id.
          :params limit: length of result returned by weapi.
          :return: a list of Song object.