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](https://github.com/michaelliao/learn-python3/blob/master/samples/oop_advance/use_enum.py)