为什么KeyError会以逻辑AND而不是逻辑OR抛出 [英] Why does a KeyError get thrown in logical AND but not in a logical OR

问题描述

我有一个特定的字符串k，它可能是或可能不是dict reversedmapping的键，还有另一个dict spendDict，它也可能是键.

I have a certain string k that may or many not a key of a dict reversedmapping, and another dict spendDict where it may also be a key.

对于不是k reversedmapping键的k值，为什么以下检查成功(即运行到完成):

Why does the following check succeed (i.e. run to completion), for a value of k that is not a key of reversedmapping:

if (k not in reversedmapping) or (reversedmapping[k] not in spendDict)

当我将其更改为逻辑AND时，我得到一个KeyError(对于k):

whereas I get a KeyError (for k) when I change it to logical AND:

if (k not in reversedmapping) and (reversedmapping[k] not in spendDict):

以及如何重写AND变体以避免KeyError?

And how do I rewrite the AND variant to avoid the KeyError?

推荐答案

如果(k not in reversedmapping)是True，则

(k not in reversedmapping) or (reversedmapping[k] not in spendDict)

短路 ，这意味着它会返回True而不进行评估

(reversedmapping[k] not in spendDict)

由于True or anything是True.

相反，

 (k not in reversedmapping) and (reversedmapping[k] not in spendDict)

必须先评估两个部分，然后Python才能确定总表达式 是True.

must evaluate both parts before Python can determine that the total expression is True.

如果(k not in reversedmapping)是True，则reversedmapping[k]将升高 KeyError.

If (k not in reversedmapping) is True, then reversedmapping[k] will raise KeyError.

如果(k not in reversedmapping)是False，则表达式短路 并返回False，因为False and anything是False.

If (k not in reversedmapping) is False, then the expression short-circuits and returns False since False and anything is False.

所以

(k not in reversedmapping) and (reversedmapping[k] not in spendDict)

将始终返回False或引发KeyError.没有任何情况 在此情况下，它可以返回True. 两个条件都不能为真.

will always either return False or raise a KeyError. There is no situation under which it could return True. Both conditions can not be True.

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