课程里面的用input的例子运行不了

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

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

# x = '???' # 用户输入的字符串

x = input("输入：")

if x in weekdays:

    print 'input ok'

else:

    print 'input error'

错误信息：

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

    x = input("输入：")

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

NameError: name 'MON' is not defined