python单元测试：对抛出AssertionError的相同对象的assertEqual

I have a class as

我有一堂课

class PlaylistManager(models.Manager):
    def add_playlist(self, name):
        playlist = Playlist(name=name)
        playlist.save()
        return playlist

    def get_playlist_with_id(self, id):
        return super(PlaylistManager, self).get_query_set().filter(pk=id)

class Playlist(models.Model):
    name = models.CharField(max_length=30)
    date_created = models.DateTimeField(auto_now_add=True)
    date_modified = models.DateTimeField(auto_now=True)
    deleted = models.BooleanField(default=False)
    objects = PlaylistManager() # is a customer manager

    def __repr__(self):
        return '<Playlist name:%s, date_created:%s, date_modified:%s, deleted:%s>' % \
                (self.name, self.date_created, self.date_modified, self.deleted)

    class Meta:
        db_table = 'playlists'
class PlaylistMana