import  dateutil

def before_month_lastday(ti):

    today=dateutil.parser.parse(str(ti))

    first = datetime.date(day=1, month=today.month, year=today.year)

    lastMonth = first - datetime.timedelta(days=1)

    cc=str(lastMonth.year)+str(lastMonth.month)+str(lastMonth.day)

    return int(cc)

print(before_month_lastday(20171011))

2017930