Python中使用枚举类

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

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)

[, , ]

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