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
class Directions:
NORTH = 1
EAST = 2
SOUTH = 3
WEST = 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
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
from enum import Enum
class Directions(Enum):
NORTH = 1
EAST = 2
SOUTH = 3
WEST = 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
3.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
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
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
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:
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
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
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
>>> # 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:
# 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:
# 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
2
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:
# 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: '**'>]
8
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