当枚举成员不存在时返回默认会员

Question

我有一个枚举，我希望当成员内部不存在默认会员。例如：

class MyEnum(enum.Enum):
    A = 12
    B = 24
    CUSTOM = 1


print(MyEnum.UNKNOWN) # Should print MyEnum.CUSTOM

我知道我可以使用类似的元素：

class MyMeta(enum.EnumMeta):
    def __getitem__(cls, name):
        try:
            return super().__getitem__(name)
        except KeyError as error:
            return cls.CUSTOM

class MyEnum(enum.Enum,metaclass=MyMeta):
    ...

但是，只有在使用myenum ['unknown']访问枚举时，这似乎是有效的。有没有办法涵盖在不存在成员时访问枚举成员的两种方法？

Answer 1

将__ getAttr __的定义添加到Metaclass：

class MyMeta(enum.EnumMeta):
    def __getitem__(cls, name):
        try:
            return super().__getitem__(name)
        except KeyError as error:
            return cls.CUSTOM
    
    def __getattr__(cls, name):
        try:
            return super().__getattr__(name)
        except AttributeError as error:
            return cls.CUSTOM

然后，您的代码将输出：

MyEnum.CUSTOM

Answer 2

编译 @brokentbenchmark 的答案和这个答案，以及一些知识，让我们使默认枚举成员一个参数：

class EnumWithDefaultMeta(enum.EnumMeta):

    @classmethod
    def __prepare__(metacls, name, bases, **kwargs):
        return super().__prepare__(name, bases)

    def __new__(metacls, name, bases, namespace, **kwargs):
        newclass = super().__new__(metacls, name, bases, namespace)
        newclass._missing_ = classmethod(metacls._missing_)
        return newclass

    def __init__(cls, name, bases, namespace, default='UNKNOWN'):
        cls.default = default
        super().__init__(name, bases, namespace)

    def __getitem__(cls, name):
        try:
            return super().__getitem__(name)
        except KeyError:
            return super().__getitem__(cls.default)

    def __getattr__(cls, name):
        try:
            # noinspection PyUnresolvedReferences
            return super().__getattr__(name)
        except AttributeError:
            # noinspection PyUnresolvedReferences
            return super().__getattr__(cls.default)

    def _missing_(cls, _value):
        return super().__getitem__(cls.default)

class MyEnum(enum.Enum, metaclass=EnumWithDefaultMeta, default='CUSTOM'):
    A = 12
    B = 24
    CUSTOM = 1

输出：

MyEnum(123)
>>> <MyEnum.CUSTOM: 1>
MyEnum.Test
>>> <MyEnum.CUSTOM: 1>
MyEnum['Test']
>>> <MyEnum.CUSTOM: 1>