def get_playlist_songs(self, playlist_id, limit=1000):
        """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.
        """

        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
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.