請輸入星期幾的第一個字母來判斷一下是星期幾，如果第一個字母一樣，則繼續判斷第二個字母。

def juge(num,week_list):

    w = input('請輸入第%s個字母:'%num)
    w = w.lower()
    res = []
    state = 0
    for week in week_list:
        if w == week[0 + num - 1: 1 + num - 1]:
            state += 1
            res.append(week)

    if state == 1:
        print('是%s'% res[0])
    elif state > 1:
        print('還要輸入一次')
        num += 1
        week_list = res
        juge(num, week_list)
    else:
        print('非法輸入')


if __name__ == '__main__':
    #  初始條件
    num = 1
    week_list = ['mon', 'tues', 'wed', 'thur', 'fri', 'sat', 'sun']
    juge(num, week_list)