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’