set的特点
weekdays = set(['MON', 'TUE', 'WEN', 'THU', 'FRI', 'SAT', 'SUN'])
x = 'MON', 'TUE', 'WEN', 'THU', 'FRI', 'SAT', 'SUN'
if x in weekdays:
print 'input ok'
else:
print 'input error'
为何输出的是 input error,if不是判断 x是否存在于WEEKDAYS中吗,那输出的不应该是input ok 才对嘛