python vs numpy中的布尔和类型检查

2022-01-19 00:00:00 python numpy pep8 boolean

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typenumpy.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 Trueif 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[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.


解决方案

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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