Question

I am trying to return a boolean in a function like this:

return mylist and any(condition(x) for x in mylist)

The behavior should be to return True if the list is empty or if any element in it meets the condition. I am using the first operand as a shortcircuit since any would return True if the list was empty, which is not what I am after.

I would expect [] and boolval to return False since the list is empty, but to my surprise it returns [] whether boolval is True or False. I would expect the first operand to be automatically evaluated as a boolean since it is involved in a comparison operation, and not whatever is happening.

I am not really asking how to solve my problem, which is easily done by an explicit type conversion: bool(mylist), but rather asking what is happening and why.

edit: when I ask "why" this is happening I am not looking for the "facts" only, as they are already explained in the linked duplicate question, but also the reasons behind the implementation of this behavior.

Answer 1

par(mfrow = c(4, 4)) for (i in c(6:17)) { print(ggpubr::ggboxplot(logdat, x = "Diagnostic", y = names(logdat)[i] , color = "Diagnostic", add = "jitter") + stat_compare_means(comparisons = my_comparisons, method = "t.test")) }和and运算符不返回or。他们返回最后评估的结果（其他动态语言也是如此，例如javascript）。

official documentation describes that

对于True/False，第一个伪造的值或最后一个操作数
用于and，第一个真实值或最后一个操作数

这是设计使然，因此您可以创建类似or的表达式。因此，如果要保证返回布尔值，则必须

return username or 'guest'

代替

return bool(x or y)

Answer 2

因为凯尔伍德说过：

x and y如果x为假，则给出x，否则给出y。

这就是重点（{and不是or :-)），所以最好的是：

return all([my_list,any(condition(x) for x in my_list)])

Answer 3

这与python如何评估表达式有关。

一个空列表被python认为是假的，这意味着'and'之后的代码将不会执行，因为这不会改变结果。

Python无需将空列表转换为bool，因为它不会与任何内容进行比较，而只是将其作为空列表返回。

这应该不会为您带来任何改变，如果您测试该函数的返回值，则将以与该函数返回False相同的方式求值。

Why does [] and bool return []?

3 个答案: