python取得指定日期的前一天或前n天

def get_yesterday(today):
    import datetime
    yesterday = datetime.datetime(int(today[:4]), int(today[4:6]), int(today[6:])) + datetime.timedelta(days=-1)
    yesterday = str(yesterday)
    return '%s%s%s' % (yesterday[:4], yesterday[5:7], yesterday[8:10])



d = get_yesterday('20170209')
则d = ‘20170208’

总感觉还有更好的方法，不过今天匆忙写了这个。