当我们需要定义常量时，一个方法是用大写变量通过整数来定义，例如月份

JAN = 1
FEB = 2
MAR = 3
APR=4
May=5
Jun=6
Jul=7
Aug=8
Sep=9
Oct=10
NOV = 11
DEC = 12

　　好处是简单，缺点是类型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，或者枚举它所有成员

>>> Month.Jan
<Month.Jan: 1>
>>> Month.Feb
<Month.Feb: 2>

　　

>>> 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
    Mon=1
    Tue=2
    Wed=3
    Thu=4
    Fri=5
    Sat=6

　　装饰器unique保证没有重复

　　访问这些枚举值有多种方法

>>> day1 = Weekday.Mon
>>> print(day1)
Weekday.Mon
>>> print(Weekday.Tue)
Weekday.Tue
>>> print(Weekday['Tue'])
Weekday.Tue
>>> print(Weekday.Tue.value)
2
>>> print(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

　　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。

　　练习

　　把Student的gender属性改为枚举类型，可以避免使用字符串

from enum import Enum, unique
class Gender(Enum):
  Male=0
  Female=1
class Student(object):
    def __init__(self, name, gender):
        self.name = name
        self.gender = gender

bart=Student('Bart',Gender.Male)