当我们需要定义常量时，一个办法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
好处是简单，缺点是类型是int，并且仍然是变量。
更好的方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类来实现这个功能：
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类型的枚举类，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)
Jan => Month.Jan , 1 Feb => Month.Feb , 2 Mar => Month.Mar , 3 Apr => Month.Apr , 4 May => Month.May , 5 Jun => Month.Jun , 6 Jul => Month.Jul , 7 Aug => Month.Aug , 8 Sep => Month.Sep , 9 Oct => Month.Oct , 10 Nov => Month.Nov , 11 Dec => Month.Dec , 12
value属性则是自动赋给成员的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
from enum import Enum, unique @unique class Weekday(Enum): Sun = 0 # Sun的value被设定为0 Mon = 1 Tue = 2 Wed = 3 Thu = 4 Fri = 5 Sat = 6
确保唯一值
@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'])
Weekday.Tue
print(Weekday.Tue.value)
2
print(Weekday(1))
Weekday.Mon
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
可见，既可以用成员名称引用枚举常量，又可以直接根据value的值获得枚举常量。
使用自动值
from enum import Enum,auto class Color(Enum): RED=auto() BLUE=auto() GREEN=auto()
迭代
list(Color)
[<Color.RED: 1>, <Color.BLUE: 2>, <Color.GREEN: 3>]
list(Color)[0].value
1
for name, member in Color.__members__.items(): print(name, member)
RED Color.RED BLUE Color.BLUE GREEN Color.GREEN
比较
Color.RED is Color.RED
True
Color.RED is Color.BLUE
False
