问题描述

我今天在 python if 子句中遇到了意想不到的结果:

I ran into unexpected results in a python if clause today:

import numpy
if numpy.allclose(6.0, 6.1, rtol=0, atol=0.5):
    print 'close enough'  # works as expected (prints message)

if numpy.allclose(6.0, 6.1, rtol=0, atol=0.5) is True:
    print 'close enough'  # does NOT work as expected (prints nothing)

经过一番探索(即这个问题，特别是this answer)，我明白了原因:numpy.allclose()返回的type是numpy.bool_ 而不是普通的旧 bool，显然如果 foo = numpy.bool_(1)，那么 if foo 将评估为 True 而 if foo 为 True 将评估为 False.这似乎是 is 运算符的工作.

After some poking around (i.e., this question, and in particular this answer), I understand the cause: the type returned by numpy.allclose() is numpy.bool_ rather than plain old bool, and apparently if foo = numpy.bool_(1), then if foo will evaluate to True while if foo is True will evaluate to False. This appears to be the work of the is operator.

我的问题是:为什么 numpy 有自己的布尔类型，鉴于这种情况，最佳实践是什么?在上面的示例中，我可以通过编写 if foo: 来获得预期的行为，但我喜欢更严格的 if foo is True: 因为它排除了像 2 和 [2] 从返回 True，有时需要显式类型检查.

My questions are: why does numpy have its own boolean type, and what is best practice in light of this situation? I can get away with writing if foo: to get expected behavior in the example above, but I like the more stringent if foo is True: because it excludes things like 2 and [2] from returning True, and sometimes the explicit type check is desirable.

推荐答案

为什么 numpy 有自己的布尔类型

why does numpy have its own boolean type

空间和速度.Numpy 将事物存储在紧凑的数组中；如果它可以将布尔值放入单个字节中，它会尝试.对于 Python 对象，您无法轻松做到这一点，因为您必须存储引用，这会显着降低计算速度.

Space and speed. Numpy stores things in compact arrays; if it can fit a boolean into a single byte it'll try. You can't easily do this with Python objects, as you have to store references which slows calculations down significantly.

我可以写 if foo: 来获得上面示例中的预期行为，但我喜欢更严格的 if foo is True: 因为它排除了像 2 和 [2] 之类的东西返回 True，有时是显式的类型检查是可取的.

I can get away with writing if foo: to get expected behavior in the example above, but I like the more stringent if foo is True: because it excludes things like 2 and [2] from returning True, and sometimes the explicit type check is desirable.

好吧，不要那样做.

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