“在"的用法和含义在 if 语句中? [英] Use and meaning of "in" in an if statement?

问题描述

在 Zed Shaw 的 Learn Python the Hard Way 示例中，其中一个练习显示了以下代码:

next = raw_input(">")如果下一个为0"或下一个为1":how_much = int(下一个)

我很难理解这个声明中 in 的含义.我习惯使用 if 语句，例如在 javascript 中，其语法类似于:

var = 5;如果(变量> 3){//要执行的代码}

这个if/in 语句(在python中)和javascript中的if()一样吗?

找到这个问题的答案很棘手，因为 in 是一个很短的字符串，无法通过搜索引擎缩小答案的范围，而且我不知道其操作的正确名称.

解决方案

这取决于 next 是什么.

如果它是一个字符串(如您的示例)，则 in 检查子字符串.

<预><代码>>>>靛蓝"中的中"真的>>>紫罗兰"中的中"错误的>>>10"中的0"真的>>>10"中的1"真的

如果它是不同类型的可迭代对象(列表、元组、集合、字典...)，则 in 检查成员资格.

<预><代码>>>>"in" in ["in", "out"]真的>>>"in" in ["靛蓝"，"紫罗兰"]错误的

在字典中，成员资格被视为键之一":

<预><代码>>>>入"入{入":出"}真的>>>入"入{出":入"}错误的

In an example from Zed Shaw's Learn Python the Hard Way, one of the exercises displays the following code:

next = raw_input("> ")
if "0" in next or "1" in next:
    how_much = int(next)

I'm having a hard time understanding the meaning of in in this statement. I'm used to using if statements, such as in javascript, where the syntax is something like:

var = 5;
if (var > 3) {
    //code  to be executed
}

Is this if / in statement (in python) the same as if() in javascript?

Finding an answer to this has been tricky because the in is such a short string to narrow down an answer via search engine, and I don't know the proper name for its operation.

解决方案

It depends on what next is.

If it's a string (as in your example), then in checks for substrings.

>>> "in" in "indigo"
True
>>> "in" in "violet"
False
>>> "0" in "10"
True
>>> "1" in "10"
True

If it's a different kind of iterable (list, tuple, set, dictionary...), then in checks for membership.

>>> "in" in ["in", "out"]
True
>>> "in" in ["indigo", "violet"]
False

In a dictionary, membership is seen as "being one of the keys":

>>> "in" in {"in": "out"}
True
>>> "in" in {"out": "in"}
False

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