是否有任何内置的 Python 值，其中真值评估无效?

Is there any builtin Python value where truth evaluation is invalid?(是否有任何内置的 Python 值，其中真值评估无效?)

问题描述

我有一组子类，它们都应该定义一个属性 x，该属性应该评估为 True 或 False.为了在我忘记在子类中设置此值时捕获错误，我想在其超类中将其设置为真值评估导致错误的值.Python 中是否有具有这种行为的内置值?我有点期望 NotImplemented 有这种行为，但它的评估结果为 True.

I have a set of sub-classes that should all define an attribute x that should either evaluate to True or False. In order to catch bugs when I forget to set this value in a subclass, I would like to set it in its superclass to a value for which truth evaluation results in an error. Is there any built-in value in Python with this behaviour? I sort of expected NotImplemented to have this behaviour, but it evaluates to True.

我可以将它设置为 numpy.array([0, 0]) 其中 if x: 引发 ValueError，但感觉不对.同样，我可以定义自己的类，其中 __bool__ 引发异常.但是是否有适合此目的的内置值?

I could set it to numpy.array([0, 0]) where if x: raises ValueError, but that feels wrong. Likewise, I could define my own class where __bool__ raises an exception. But is there any built-in value appropriate for this purpose?

其他选择是设置一个属性(抽象与否)或根本不定义它(所以我们获取 AttributeError).

Other alternatives would be to set a property (abstract or not) or to not define it at all (so we get AttributeError).

推荐答案

以后，NotImplemented 将在布尔上下文中无效.

In the future, NotImplemented will be invalid in a boolean context.

从 Python 3.9 开始，不推荐在布尔上下文中使用 NotImplemented.来自文档:

Starting with Python 3.9, using NotImplemented in a boolean context is deprecated. From the documentation:

不推荐在布尔上下文中评估 NotImplemented.虽然它当前评估为 true，但它会发出 DeprecationWarning.它将在 Python 的未来版本中引发 TypeError.

Evaluating NotImplemented in a boolean context is deprecated. While it currently evaluates as true, it will emit a DeprecationWarning. It will raise a TypeError in a future version of Python.

截至 2020 年 10 月 6 日，Python 的未来版本(据我所知)尚未确定，但在未来某个时间点(我预计不会在 Python 3.11 之前)，未实现 将成为真值评估无效的 Python 内置值.

As of 2020-10-06, that future version of Python has (as far as I know) not been determined, but at some point in the future (I'd expect not before Python 3.11), NotImplemented will become a builtin Python value where truth evaluation is invalid.

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