python基础——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 EnumMonth = 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：

#python 枚举类示例
#2016-8-30 18:55:25
#MengmengCoding
# -*- coding: utf-8 -*-from enum import Enum#Enum是一个枚举类
#Month,Weekday各为一个Enum类的实例化
Month=Enum('Month',('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
Weekday=Enum('Weekday',('Mon','Tue','Wed','Thu','Fri','Sat','Sun'))for name,member in Month.__members__.items():
print(name,'=>',member,',',member.value)print('--------cut-off line--------')
#输出：
'''
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
'''for name,member in Weekday.__members__.items():
print(name,'=>',member,',',member.value)print('--------cut-off line--------')
#输出：
'''
Mon => Weekday.Mon , 1
Tue => Weekday.Tue , 2
Wed => Weekday.Wed , 3
Thu => Weekday.Thu , 4
Fri => Weekday.Fri , 5
Sat => Weekday.Sat , 6
Sun => Weekday.Sun , 7
'''#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
#这里还是用周来举例吧，谁让我见识少呢~
#用WeekDay来区别之前的Weekday吧 from enum import Enum,unique@unique #@unique装饰器可以帮助我们检查保证没有重复值
class WeekDay(Enum): #由Enum类派生得到
Sun=0
Mon=1
Tue=2
Wed=3
Thu=4
Fri=5
Sat=6day1=WeekDay.Mon print('day1=',day1)
print('WeekDay.Tue=',WeekDay.Tue)
print('WeekDay[\'Tue\']=',WeekDay['Tue'])
print('WeekDay.Tue.value=',WeekDay.Tue.value)
print('day1==WeekDay.Mon ?',day1==WeekDay.Mon)
print('day1==WeekDay.Tue ?',day1==WeekDay.Tue)
print('day1==WeekDay(1) ?',day1==WeekDay(1))#输出：
'''
day1= WeekDay.Mon
WeekDay.Tue= WeekDay.Tue
WeekDay['Tue']= WeekDay.Tue
WeekDay.Tue.value= 2
day1==WeekDay.Mon ? True
day1==WeekDay.Tue ? False
day1==WeekDay(1) ? True
'''print('--------cut-off line--------')for name,member in WeekDay.__members__.items():
print(name,'=>',member)#输出：
'''
Sun => WeekDay.Sun
Mon => WeekDay.Mon
Tue => WeekDay.Tue
Wed => WeekDay.Wed
Thu => WeekDay.Thu
Fri => WeekDay.Fri
Sat => WeekDay.Sat
'''