Python打印筛选器中不存在的值

我有一些过滤邮箱的代码，以查看是否存在某些主题。如何打印查询集中不存在的主题

这是我的密码：

query = (Q(subject__icontains='Backup - no index - Friday') | Q(subject__icontains='backup - with no index Thursday'))

if not testfolder.filter(query, datetime_received__gt=since, sender='email@email.dk').exists():

    print('we are missing a backup')
else:
    print('email found')