課程里面的用input的例子運行不了

# -*- coding: utf-8 -*-

weekdays = set(['MON', 'TUE', 'WED', 'THU', 'FRI', 'SAT', 'SUN'])

# x = '???' # 用戶輸入的字符串

x = input("輸入：")

if x in weekdays:

? ? print 'input ok'

else:

? ? print 'input error'

錯誤信息：

? File "C:/Users/YCKJ1005/PycharmProjects/Demo0821/try.py", line 4, in <module>

? ? x = input("輸入：")

? File "<string>", line 1, in <module>

NameError: name 'MON' is not defined