0 == False但是[]！= False？ [英] 0 == False but [] != False?

问题描述

这是一个稍微天真的问题，但我知道0可用于

代表False。所以

>> 0 == False

True


但是，我知道我可以使用[]代表False，如

>> if not []：print''empty''

....

空


但是接下来的做法令人惊讶（对我来说！ ）结果

>> [] == False

False


有人能指出为什么会这样吗？


谢谢，

Rajarshi

解决方案

Rajarshi写道：

这是一个稍微天真的问题，但我知道0可用于

代表False。所以

>>> 0 == False

正确


但是，我知道我可以使用[]代表False，如

>>>如果不是[]：打印''空''

...

空


然后做以下操作会给我一个惊喜（对我来说！）结果

>>> [] ==错误

错误


可能有人指出为什么会这样吗？

" if foo："不检查foo == True是否或foo == False而是

bool（foo）。对于空列表，字符串，元组，词汇和其他一些东西，

bool（foo）== False，而对于列表等，至少有一个元素，

" bool（foo）== True"。


-

Robert Kern


我已经开始相信整个世界都是一个谜，一个无害的谜团

由于我们自己疯狂地试图解释它而使它变得可怕，好像它已经

一个潜在的事实。

- Umberto Eco

Rajarshi写道：

这是一个稍微天真的问题，但我知道0可用于

代表False。所以

>>>> 0 == False

True


但是，我知道我可以使用[]代表False，如

>>>>如果不是[]：打印''空''

...

空


但是接下来做了一个令人惊讶的（对我来说！）结果

>>>> [] == False

False


有人能指出为什么会这样吗？


谢谢，

Rajarshi

打坐：


pyisinstance（False，int）

真

pyisinstance（[]，int）

False

pybool（[]）

False


James

> [] == False

False


有人能指出为什么会这样吗？

写作，if x是写作的缩写如果bool（x）。

评估bool（x）检查x .__非零__（）

如果该方法不是''定义后，它会检查

x .__ len __（）以查看x是否为非空容器。


在您的情况下，写入if []"转换为

" if len（[]）！= 0"，其评估结果为False。


正确和错误的类型为bool，这是一个子类

of int。所以，False确实等于零和

真实等于一。


相反，空列表不是int类型。 />
所以[]！= False eventHough bool（[]）== False。

Raymond

This is a slightly naive question, but I know that 0 can be used to
represent False. So

>>0 == False

True

But, I know I can use [] to represent False as in

>>if not []: print ''empty''

....
empty

But then doing the following gives a surprising (to me!) result

>>[] == False

False

Could anybody point out why this is the case?

Thanks,
Rajarshi

解决方案

Rajarshi wrote:

This is a slightly naive question, but I know that 0 can be used to
represent False. So

>>>0 == False

True

But, I know I can use [] to represent False as in

>>>if not []: print ''empty''

...
empty

But then doing the following gives a surprising (to me!) result

>>>[] == False

False

Could anybody point out why this is the case?

"if foo:" does not check if "foo == True" or "foo == False" but rather
"bool(foo)". For empty lists, strings, tuples, dicts and some other things,
"bool(foo) == False", while for lists, etc., with at least one element,
"bool(foo) == True".

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

Rajarshi wrote:

This is a slightly naive question, but I know that 0 can be used to
represent False. So

>>>>0 == False

True

But, I know I can use [] to represent False as in

>>>>if not []: print ''empty''

...
empty

But then doing the following gives a surprising (to me!) result

>>>>[] == False

False

Could anybody point out why this is the case?

Thanks,
Rajarshi

Meditate on:

pyisinstance(False, int)
True
pyisinstance([], int)
False
pybool([])
False

James

>[] == False

False

Could anybody point out why this is the case?

Writing, "if x" is short for writing "if bool(x)".
Evaluating bool(x) checks for a x.__nonzero__()
and if that method isn''t defined, it checks for
x.__len__() to see if x is a non-empty container.

In your case, writing "if []" translates to
"if len([]) != 0", which evaluates to False.

True and False are of type bool which is a subclass
of int. So, False really is equal to zero and
True really is equal to one.

In contrast, the empty list is not of type int.
So [] != False eventhough bool([]) == False.
Raymond

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