35.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)

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

@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

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。