本篇主要介绍Python中枚举类的用法，更多内容请参考:Python学习指南
当我们需要定义常量时，一个办法就是用大写变量通过整数来定义，例如月份：
JAN = 1
FEB = 2
MAR = 3
···
NOV = 11
DEC = 12
好处就是简单，确定是类型是int，并且仍然是变量。
更好的办法是为这样的枚举类型定义一个class类型，然后，每个常量都是class的一个唯一实例。Python提供了Enum来来实现这个功能：
from enum import Enum
Month = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
这样我们就获得了Month类型的枚举值，可以直接使用Month.Jan来引用一个常量，或者枚举它的所有成员：
for name, member in Month.__members__.items():
print(name, '=>', member, ',', member.value)
value属性则是自动赋给成员int常量，默认从1开始计数。
如果需要更精确地控制枚举类型，可以从Enum派生出自定义类：
@unique
class Weekday(Enum):
Sun = 0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
@unique装饰器可以帮助我们检查保证没有重复值
访问这些枚举类型可以有若干种方法：
>>> day1 = Weekday.Mon
>>> print(day1)
Weekday.Mon
>>> print(Weekday.Tue)
Weekday.Tue
>>> print(Weekday['Tue'].name)
Tue
>>> print(Weekday.Tue.value)
2
>>> print(day1 == Weekday.Mon)
True
>>> print(day1 == Weekday.Tue)
False
>>> print(Weekday(1))
Weekday.Mon
>>> print(day1 == Weekday(1))
True
>>> Weekday(7)
Traceback (most recent call last):
...
ValueError: 7 is not a valid Weekday
>>> for name, member in Weekday.__members__.items():
... print(name, '=>', member)
...
Sun => Weekday.Sun
Mon => Weekday.Mon
Tue => Weekday.Tue
Wed => Weekday.Wed
Thu => Weekday.Thu
Fri => Weekday.Fri
Sat => Weekday.Sat
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