带有"== True"的表达式和“是真实的"；给出不同的结果 [英] Expressions with "== True" and "is True" give different results

问题描述

我有以下 MCVE :

#!/usr/bin/env python3                                           

import pandas as pd

df = pd.DataFrame([True, False, True])

print("Whole DataFrame:")
print(df)

print("\nFiltered DataFrame:")
print(df[df[0] == True])

我期望的输出如下:

Whole DataFrame:
     0
  0  True
  1  False
  2  True

Filtered DataFrame:
     0
  0  True
  2  True

好的，但是 PEP8 样式似乎是错误的，它说: E712与True的比较应为if cond is True或if cond .所以我将其更改为is True而不是== True，但现在失败了，输出为:

Okay, but the PEP8 style seems to be wrong, it says: E712 comparison to True should be if cond is True or if cond. So I changed it to is True instead of == True but now it fails, the output is:

Whole DataFrame:
    0
0   True
1  False
2   True

Filtered DataFrame:
0     True
1    False
2     True
Name: 0, dtype: bool

这是怎么回事?

推荐答案

这里的问题是，在df[df[0] == True]和中，您没有将对象与True 进行比较.

The catch here is that in df[df[0] == True], you are not comparing objects to True.

正如其他答案所说，==在pandas中重载以生成Series而不是通常的bool. []也被重载，以解释Series并给出过滤后的结果.该代码本质上等效于:

As the other answers say, == is overloaded in pandas to produce a Series instead of a bool as it normally does. [] is overloaded, too, to interpret the Series and give the filtered result. The code is essentially equivalent to:

series = df[0].__eq__(True)
df.__getitem__(series)

因此，您不会在此处留下==来违反PEP8.

So, you're not violating PEP8 by leaving == here.

本质上，pandas提供了熟悉的语法不同寻常的语义-这就是造成混乱的原因.

Essentially, pandas gives familiar syntax unusual semantics - that is what caused the confusion.

根据Stroustroup (sec.3.3.3)，操作员重载已引起麻烦自从它发明以来就一直对此保持关注(他不得不认真考虑是否将其包含在C ++中). 在C ++中看到了更多滥用情况，戈斯林在Java中遇到了另一种极端情况，禁止完全如此，事实证明那是一个极端.

Accoring to Stroustroup (sec.3.3.3), operator overloading has been causing trouble due to this ever since its invention (and he had to think hard whether to include it into C++). Seeing even more abuse of it in C++, Gosling ran to the other extreme in Java, banning it completely, and that proved to be exactly that, an extreme.

结论是，现代语言和代码趋向于具有运算符重载，但要密切注意不要过度使用它，并使语义保持一致.

As a conclusion, modern languages and code tend to have operator overloading but watch closely not to overuse it and for semantics to stay consistent.

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