Python3之使用枚举类

　　当我们需要定义常量时，一个方法是用大写变量通过整数来定义，例如月份

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JAN = 1 FEB = 2 MAR = 3 APR=4 May=5 Jun=6 Jul=7 Aug=8 Sep=9 Oct=10 NOV = 11 DEC = 12

　　好处是简单，缺点是类型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：

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1 2	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，或者枚举它所有成员

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>>> Month.Jan <Month.Jan: 1> >>> Month.Feb <Month.Feb: 2>

　　

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>>> 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

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from enum import Enum,unique @unique class Weekday(Enum):     Sun=0     Mon=1     Tue=2     Wed=3     Thu=4     Fri=5     Sat=6

　　装饰器unique保证没有重复

　　访问这些枚举值有多种方法

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>>> 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。

　　练习

　　把Student的gender属性改为枚举类型，可以避免使用字符串

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from enum import Enum, unique class Gender(Enum):   Male=0   Female=1 class Student(object):     def __init__(self, name, gender):         self.name = name         self.gender = gender   bart=Student('Bart',Gender.Male)