當我們需要定義常量時，一個方法是用大寫變量通過整數來定義，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
好處是簡單，缺點是類型int，并且仍然是變量。
更好的方法是為這樣的枚舉類型定義一個class類型，然後，每個常量都是class的一個唯一執行個體。Python提供了Enum類來實作這個功能：
from enum import Enum
Month=Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
這樣我們就獲得了Month類型的枚舉類，可以直接使用Month.Jan來引用一個常量，或者枚舉它所有成員
>>> Month.Jan
>>> Month.Feb
>>> 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屬性則是自動賦給成員的int常量，預設從1開始計數
如果需要更精确地控制枚舉類型，可以從Enum派生出自定義類
from enum import Enum,unique
@unique
class Weekday(Enum):
Sun=0
Mon=1
Tue=2
Wed=3
Thu=4
Fri=5
Sat=6
裝飾器unique保證沒有重複
通路這些枚舉值有多種方法
>>> 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
可見，既可以用成員名稱引用枚舉常量，又可以直接根據value的值獲得枚舉常量。
練習
把Student的gender屬性改為枚舉類型，可以避免使用字元串
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)