Python enum的使用总结

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

class Directions:NORTH = 1EAST = 2SOUTH = 3WEST = 4

使用成员

Direction.EAST 
Direction.SOUTH 

检查成员

>>> 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

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。

enum的定义/声明

from enum import Enumclass Directions(Enum):NORTH = 1EAST = 2SOUTH = 3WEST = 4

使用和类型检查：

>>> 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

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：

>>> Directions(5)
ValueError: 5 is not a valid Directions

遍历成员

>>> for name, value in Directions.__members__.items():
...     print(name, value)
...
NORTH Directions.NORTH
EAST Directions.EAST
SOUTH Directions.SOUTH
WEST Directions.WEST

在继承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

from enum import Enumclass Directions(Enum):NORTH = 1EAST = 2SOUTH = 3WEST = 4def angle(self):right_angle = 90.0return right_angle * (self.value - 1)@staticmethoddef 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

将Enum类属性的值定义为函数或方法

from enum import Enum
from functools import partialdef plus_90(value):return Directions(value).angle + 90class Directions(Enum):NORTH = 1EAST = 2SOUTH = 3WEST = 4PLUS_90 = partial(plus_90)def __call__(self, *args, **kwargs):return self.value(*args, **kwargs)@propertydef angle(self):right_angle = 90.0return 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

class TimeUnit(Enum):MONTH = "MONTH"WEEK = "WEEK"DAY = "DAY"HOUR = "HOUR"MINUTE = "MINUTE"@classmethoddef _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

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

class TimeUnit(Enum):MONTH = "MONTH"WEEK = "WEEK"DAY = "DAY"HOUR = "HOUR"MINUTE = "MINUTE"@classmethoddef _missing_(cls, value: str):raise Exception("Customized exception")print(TimeUnit("MONTH"))
TimeUnit("abc")

输出

TimeUnit.MONTHValueError: '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

from enum import Enum, EnumMeta
from functools import partialclass 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 valueclass 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.A1123...
Exception: Customized exception

第二种如果成员找不到时不需要自定义的报错也有一种简单的方法，即直接使用__getitem__方法：

class SomeEnum(CaseInsensitiveEnum):A1 = partial(lambda x: x)def __call__(self, *args, **kwargs):return self.value(*args, **kwargs)if __name__ == '__main__':s = SomeEnum.__getitem__("A1")print(s("a"))      

输出

a

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

>>> # 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：

# enum_test.py
from enum import Enumclass Arithmetic(Enum):ADD = "+"SUB = "-"MUL = "*"DIV = "/"

# functional_api_test.py
from enum import EnumDynamicEnum = Enum("Arithmetic", {"MOD": "%"}, module="enum_test", qualname="enum_test.Arithmetic")print(DynamicEnum.MOD)print(eval(f"5 {DynamicEnum.MOD.value} 3"))

Arithmetic.MOD
2

# functional_api_test.py
from aenum import extend_enum
from enum_test import Arithmeticextend_enum(Arithmetic, "EXP", "**")
print(Arithmetic, list(Arithmetic))
print(eval(f"2 {Arithmetic.EXP.value} 3"))

<enum 'Arithmetic'> [<Arithmetic.ADD: '+'>, <Arithmetic.SUB: '-'>, <Arithmetic.MUL: '*'>, <Arithmetic.DIV: '/'>, <Arithmetic.EXP: '**'>]
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