为什么“[] == False"当“if not []"时评估为 False成功了吗? [英] Why does "[] == False" evaluate to False when "if not []" succeeds?

问题描述

我问这个是因为我知道检查列表是否为空的pythonic方法如下:

my_list = []如果不是 my_list:打印计算机说不"别的:# my_list 不为空打印计算机说是"

将打印计算机说不，等等.所以这导致我用False真值识别[]；但是，如果我尝试直接"比较 [] 和 False，则会得到以下结果:

<预><代码>>>>my_list == False错误的>>>my_list 是假的错误的>>>[] == 错误错误的

等等……

这是怎么回事?我觉得我错过了一些非常明显的东西.

解决方案

if 语句在布尔上下文中评估所有内容，就像对 bool() 内置函数.

以下是您实际检查 if 语句将如何评估事物的方法:

<预><代码>>>>布尔([])错误的>>>bool([]) == False真的

请参阅关于真值测试的文档，考虑空列表false，但这并不意味着它们等同于 False.

PEP 285 也有一些关于为什么以这种方式实现的很好的信息，请参阅Resolved Issues 部分中的最后一项，用于专门处理 x == True 和 x == False 的部分.

对我来说最有说服力的方面是 == 通常是可传递的，所以 a == b 和 b == c 意味着 <代码>a == c.因此，如果这是您期望的方式并且 [] == False 为真且 '' == False 为真，那么人们可能会假设 [] =='' 应该是真的(即使它显然不应该在没有隐式类型转换的语言中).

I'm asking this because I know that the pythonic way to check whether a list is empty or not is the following:

my_list = []
if not my_list:
    print "computer says no"
else:
    # my_list isn't empty
    print "computer says yes"

will print computer says no, etc. So this leads me to identify [] with False truth-values; however, if I try to compare [] and False "directly", I obtain the following:

>>> my_list == False
False
>>> my_list is False
False
>>> [] == False
False

etc...

What's going on here? I feel like I'm missing something really obvious.

解决方案

The if statement evaluates everything in a Boolean context, it is like there is an implicit call to the bool() built-in function.

Here is how you would actually check how things will be evaluated by an if statement:

>>> bool([])
False
>>> bool([]) == False
True

See the documentation on Truth Value Testing, empty lists are considered false, but this doesn't mean they are equivalent to False.

PEP 285 also has some excellent information on why it was implemented this way, see the very last bullet in the Resolved Issues section for the part that deals with x == True and x == False specifically.

The most convincing aspect to me is that == is generally transitive, so a == b and b == c implies a == c. So if it were the way you expected and [] == False were true and '' == False were true, one might assume that [] == '' should be true (even though it obviously should not be in a language without implicit type conversion).

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