前言:
枚举(enumeration
)在许多编程语言中常被表示为一种基础的数据结构使用,枚举帮助组织一系列密切相关的成员到同一个群组机制下,一般各种离散的属性都可以用枚举的数据结构定义,比如颜色、季节、国家、时间单位等
在Python中没有内置的枚举方法,起初模仿实现枚举属性的方式是
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class Directions: NORTH = 1 EAST = 2 SOUTH = 3 WEST = 4 |
使用成员:
Direction.EAST
Direction.SOUTH
检查成员:
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>>> print ( "North的类型:" , type (Direction.NORTH)) >>> print ( isinstance (Direction.EAST, Direction)) North的类型: < class 'int' > False |
成员NORTH的类型是int,而不是Direction
,这个做法只是简单地将属性定义到类中
Python
标准库enum实现了枚举属性的功能,接下来介绍enum的在实际工作生产中的用法
1.为什么要用enum,什么时候使用enum?
enum
规定了一个有限集合的属性,限定只能使用集合内的值,明确地声明了哪些值是合法值,,如果输入不合法的值会引发错误,只要是想要从一个限定集合取值使用的方式就可以使用enum
来组织值。
2.enum的定义/声明
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from enum import Enum class Directions(Enum): NORTH = 1 EAST = 2 SOUTH = 3 WEST = 4 |
使用和类型检查:
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>>> Directions.EAST <Directions.EAST: 2 > >>> Directions.SOUTH <Directions.SOUTH: 3 > >>> Directions.EAST.name 'EAST' >>> Directions.EAST.value 2 >>> print ( "South的类型:" , type (Directions.SOUTH)) South的类型: <enum 'Directions' > >>> print ( isinstance (Directions.EAST, Directions)) True >>> |
检查示例South
的的类型,结果如期望的是Directions
。name
和value
是两个有用的附加属性。
实际工作中可能会这样使用:
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fetched_value = 2 # 获取值 if Directions(fetched_value) is Directions.NORTH: ... elif Directions(fetched_value) is Directions.EAST: ... else : ... |
输入未定义的值时:
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>>> Directions( 5 ) ValueError: 5 is not a valid Directions |
3.遍历成员
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>>> for name, value in Directions.__members__.items(): ... print (name, value) ... NORTH Directions.NORTH EAST Directions.EAST SOUTH Directions.SOUTH WEST Directions.WEST |
4.继承Enum的类中定义方法
可以用于将定义的值转换为获取需要的值
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from enum import Enum class Directions(Enum): NORTH = 1 EAST = 2 SOUTH = 3 WEST = 4 def angle( self ): right_angle = 90.0 return right_angle * ( self .value - 1 ) @ staticmethod def angle_interval(direction0, direction1): return abs (direction0.angle() - direction1.angle()) >>> east = Directions.EAST >>> print ( "SOUTH Angle:" , east.angle()) SOUTH Angle: 90.0 >>> west = Directions.WEST >>> print ( "Angle Interval:" , Directions.angle_interval(east, west)) Angle Interval: 180.0 |
5.将Enum类属性的值定义为函数或方法
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from enum import Enum from functools import partial def plus_90(value): return Directions(value).angle + 90 class Directions(Enum): NORTH = 1 EAST = 2 SOUTH = 3 WEST = 4 PLUS_90 = partial(plus_90) def __call__( self , * args, * * kwargs): return self .value( * args, * * kwargs) @ property def angle( self ): right_angle = 90.0 return right_angle * ( self .value - 1 ) print (Directions.NORTH.angle) print (Directions.EAST.angle) south = Directions( 3 ) print ( "SOUTH angle:" , south.angle) print ( "SOUTH angle plus 90: " , Directions.PLUS_90(south.value)) |
输出:
0.0
90.0
SOUTH angle: 180.0
SOUTH angle plus 90: 270.0
key: 1.将函数方法用partial包起来;2.定义__call__
方法。
忽略大小写:
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class TimeUnit(Enum): MONTH = "MONTH" WEEK = "WEEK" DAY = "DAY" HOUR = "HOUR" MINUTE = "MINUTE" @ classmethod def _missing_( cls , value: str ): for member in cls : if member.value = = value.upper(): return member print (TimeUnit( "MONTH" )) print (TimeUnit( "Month" )) |
继承父类Enum
的_missing_
方法,在值的比较时将case改为一致即可
输出:
TimeUnit.MONTH
TimeUnit.MONTH
6.自定义异常处理
第一种,执行SomeEnum
(“abc”)时想要引发自定义错误,其中"abc"是未定义的属性值
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class TimeUnit(Enum): MONTH = "MONTH" WEEK = "WEEK" DAY = "DAY" HOUR = "HOUR" MINUTE = "MINUTE" @ classmethod def _missing_( cls , value: str ): raise Exception( "Customized exception" ) print (TimeUnit( "MONTH" )) TimeUnit( "abc" ) |
输出:
TimeUnit.MONTH
ValueError: 'abc' is not a valid TimeUnit
...
Exception: Customized exception
第二种:执行SomeEnum.__getattr__
(“ABC”)时,想要引发自定义错误,其中"ABC"是未定义的属性名称,需要重写一下EnumMeta中的__getattr__方法,然后指定实例Enum对象的的metaclass
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from enum import Enum, EnumMeta from functools import partial class SomeEnumMeta(EnumMeta): def __getattr__( cls , name: str ): value = cls .__members__.get(name.upper()) # (这里name是属性名称,可以自定义固定传入大写(或小写),对应下面的A1是大写) if not value: raise Exception( "Customized exception" ) return value class SomeEnum1(Enum, metaclass = SomeEnumMeta): A1 = "123" class SomeEnum2(Enum, metaclass = SomeEnumMeta): A1 = partial( lambda x: x) def __call__( self , * args, * * kwargs): return self .value( * args, * * kwargs) print (SomeEnum1.__getattr__( "A1" )) print (SomeEnum2.__getattr__( "a1" )( "123" )) print (SomeEnum2.__getattr__( "B" )( "123" )) |
输出:
SomeEnum1.A1
123
...
Exception: Customized exception
7.enum的进阶用法
Functional APIs
动态创建和修改Enum对象,可以在不修改原定义好的Enum类的情况下,追加修改,这里借用一个说明示例,具体的场景使用案例可以看下面的场景举例
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>>> # Create an Enum class using the functional API ... DirectionFunctional = Enum( "DirectionFunctional" , "NORTH EAST SOUTH WEST" , module = __name__) ... # Check what the Enum class is ... print (DirectionFunctional) ... # Check the items ... print ( list (DirectionFunctional)) ... print (DirectionFunctional.__members__.items()) ... <enum 'DirectionFunctional' > [<DirectionFunctional.NORTH: 1 >, <DirectionFunctional.EAST: 2 >, <DirectionFunctional.SOUTH: 3 >, <DirectionFunctional.WEST: 4 >] dict_items([( 'NORTH' , <DirectionFunctional.NORTH: 1 >), ( 'EAST' , <DirectionFunctional.EAST: 2 >), ( 'SOUTH' , <DirectionFunctional.SOUTH: 3 >), ( 'WEST' , <DirectionFunctional.WEST: 4 >)]) >>> # Create a function and patch it to the DirectionFunctional class ... def angle(DirectionFunctional): ... right_angle = 90.0 ... return right_angle * (DirectionFunctional.value - 1 ) ... ... ... DirectionFunctional.angle = angle ... ... # Create a member and access its angle ... south = DirectionFunctional.SOUTH ... print ( "South Angle:" , south.angle()) ... South Angle: 180.0 |
注:这里没有使用类直接声明的方式来执行枚举(定义时如果不指定值默认是从1开始的数字,也就相当于NORTH = auto(),auto是enum中的方法),仍然可以在后面为这个动态创建的DirectionFunctional
创建方法,这种在运行的过程中修改对象的方法也就是python
的monkey patching
。
Functional APIs的用处和使用场景举例:
在不修改某定义好的Enum类的代码块的情况下,下面示例中是Arithmethic
类,可以认为是某源码库我们不想修改它,然后增加这个Enum类的属性,有两种方法:
1.enum.Enum对象的属性不可以直接被修改,但我们可以动态创建一个新的Enum类,以拓展原来的Enum对象
例如要为下面的Enum对象Arithmetic增加一个取模成员MOD="%",但是又不能修改Arithmetic类中的代码块:
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# enum_test.py from enum import Enum class Arithmetic(Enum): ADD = "+" SUB = "-" MUL = "*" DIV = "/" |
就可以使用enum的Functional APIs方法:
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# functional_api_test.py from enum import Enum DynamicEnum = Enum( "Arithmetic" , { "MOD" : "%" }, module = "enum_test" , qualname = "enum_test.Arithmetic" ) print (DynamicEnum.MOD) print ( eval (f "5 {DynamicEnum.MOD.value} 3" )) |
输出:
Arithmetic.MOD
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注意:动态创建Enum对象时,要指定原Enum类所在的module名称: "Yourmodule",否则执行时可能会因为找不到源无法解析,qualname要指定类的位置:"Yourmodule.YourEnum",值用字符串类型
2.使用aenum.extend_enum可以动态修改enum.Enum对象
为enum.Enum
类Arithmetic
增加一个指数成员EXP="**",且不修改原来的Arithmetic类的代码块:
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# functional_api_test.py from aenum import extend_enum from enum_test import Arithmetic extend_enum(Arithmetic, "EXP" , "**" ) print (Arithmetic, list (Arithmetic)) print ( eval (f "2 {Arithmetic.EXP.value} 3" )) |
输出:
<enum 'Arithmetic'> [<Arithmetic.ADD: '+'>, <Arithmetic.SUB: '-'>, <Arithmetic.MUL: '*'>, <Arithmetic.DIV: '/'>, <Arithmetic.EXP: '**'>]
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原文链接:https://blog.csdn.net/Moelimoe/article/details/121435602