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使用枚举类

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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。

小结

Enum可以把一组相关常量定义在一个class中,且class不可变,而且成员可以直接比较。

参考源码

use_enum.py

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