python中的枚举类

1. 每个常量都是class的唯一一个实列

   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
   

   个人理解是枚举常量就是Class.name,可以通过名称来获取，也可以通过value来获取它