python波浪号一元运算符作为否定numpy bool数组 [英] python tilde unary operator as negation numpy bool array

问题描述

应该是一个简单的问题，但我无法在任何地方找到答案.python 中的 ~ 运算符被记录为按位反转运算符.美好的.不过，我注意到了看似精神分裂的行为，即:

Should be a simple question, but I'm unable to find an answer anywhere. The ~ operator in python is a documented as a bitwise inversion operator. Fine. I have noticed seemingly schizophrenic behavior though, to wit:

~True -> -2
~1 -> -2
~False -> -1
~0 -> -1
~numpy.array([True,False],dtype=int) -> array([-2,-1])
~numpy.array([True,False],dtype=bool) -> array([False,True])

在前 4 个示例中，我可以看到 python 正在实现(如文档所述)~x = -(x+1)，输入被视为 int 即使它是布尔.因此，对于标量布尔值，~ 不被视为逻辑否定.并不是说在使用 int 类型的布尔值定义的 numpy 数组上的行为是相同的.

In the first 4 examples, I can see that python is implementing (as documented) ~x = -(x+1), with the input treated as an int even if it's boolean. Hence, for a scalar boolean, ~ is not treated as a logical negation. Not that the behavior is identical on a numpy array defined with boolean values by with an int type.

为什么 ~ 然后在布尔数组上用作逻辑否定运算符(另请注意:~numpy.isfinite(numpy.inf) -> True?)?

Why does ~ then work as a logical negation operator on a boolean array (Also notice: ~numpy.isfinite(numpy.inf) -> True?)?

我必须在标量上使用 not() 非常烦人，但是 not() 不能用于否定数组.那么对于一个数组，我必须使用 ~，但是 ~ 不能用来否定一个标量...

It is extremely annoying that I must use not() on a scalar, but not() won't work to negate an array. Then for an array, I must use ~, but ~ won't work to negate a scalar...

推荐答案

not 是通过 __nonzero__ 特殊方法实现的，需要返回 True 或 False，因此无法给出所需的结果.而是使用了 ~ 运算符，它通过 __not__ 特殊方法实现.出于同样的原因，& 和 | 用于代替 and 和 or.

not is implemented through the __nonzero__ special method, which is required to return either True or False, so it can't give the required result. Instead the ~ operator is used, which is implemented through the __not__ special method. For the same reason, & and | are used in place of and and or.

PEP 335 旨在允许布尔运算符的重载，但由于开销过大而被拒绝(它例如会使 if 语句复杂化).PEP 225 建议了元素"运算符的通用语法，这将提供更通用的解决方案，但已延期.看来目前的情况虽然尴尬，但还不足以迫使改变.

PEP 335 aimed to allow overloading of boolean operators but was rejected because of excessive overhead (it would e.g. complicate if statements). PEP 225 suggests a general syntax for "elementwise" operators, which would provide a more general solution, but has been deferred. It appears that the current situation, while awkward, is not painful enough to force change.

np.isfinite 在标量上调用时返回 np.bool_ 类型的值，而不是 bool.np.bool_ 也是从 bool dtype 数组中提取标量值时获得的类型.如果您使用 np.True_ 和 np.False_ 代替 True 和 False，您将在下获得一致的行为~.

np.isfinite when called on a scalar returns a value of type np.bool_, not bool. np.bool_ is also the type you get when extracting a scalar value from an array of bool dtype. If you use np.True_ and np.False_ in place of True and False you will get consistent behaviour under ~.

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