获取指定日期的上一个月日期

输入：20170320，输出：20170220

import datetime
def get_lastmonth():
    date = raw_input('please input a date:\n')
    if len(date) == 8:
        newdate = date[0:4] + '-' + date[4:6] + '-' + date[6:]
        datestr = datetime.datetime.strptime(newdate, "%Y-%m-%d").date()
        year = datestr.year
        month = datestr.month
        day = datestr.day
        if month == 1:
            month = 12
            year -= 1
        else:
            month -= 1
        lastmonth = datetime.datetime(year, month, day).strftime("%Y-%m-%d")
        final = ''.join(lastmonth.split('-'))
        return final
    else:
        return None
        
print get_lastmonth()