有没有办法为python枚举指定默认值？ [英] Is there a way to specify a default value for python enums?

问题描述

给出以下枚举：

class MyEnum(IntEnum):

    A = 0
    B = 1
    C = 2

如何指定默认值。我希望能够做到：

How can I specify a default value. I want to be able to do:

my_val = MyEnum()

并具有 my_val 为< MyEnum.A：0>

这可能吗？我尝试自定义 __ new __ ， __ init __ ， __ call __ ，但是我无法使其正常工作。

Is this possible? I've tried customizing __new__, __init__, __call__ but I can't get it to work.

推荐答案

MyEnum（..）由 EnumMeta .__ call __ 。您需要重写该方法：

MyEnum(..) is handled by EnumMeta.__call__. You need to override that method:

from enum import EnumMeta, IntEnum


class DefaultEnumMeta(EnumMeta):
    default = object()

    def __call__(cls, value=default, *args, **kwargs):
        if value is DefaultEnumMeta.default:
            # Assume the first enum is default
            return next(iter(cls))
        return super().__call__(value, *args, **kwargs)
        # return super(DefaultEnumMeta, cls).__call__(value, *args, **kwargs) # PY2


class MyEnum(IntEnum, metaclass=DefaultEnumMeta):
    # __metaclass__ = DefaultEnumMeta  # PY2 with enum34
    A = 0
    B = 1
    C = 2


assert MyEnum() is MyEnum.A
assert MyEnum(0) is MyEnum.A
assert MyEnum(1) is not MyEnum.A

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