从零开始学Python-Day46-面向对象高级编程-使用枚举类

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
>>> Mouth = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
>>> for name, member in Mouth.__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
>>> 

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，如上。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

@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(day1 == Weekday.Mon)
True
>>> print(day1 == Weekday.Tue)
False
>>> print(Weekday(1))
Weekday.Mon
>>> print(day1 == Weekday(1))
True
>>> 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。