下划线_作为Python中的变量名[重复]

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What is the purpose of the single underscore “_” variable in Python? 4 answers
Python中单个下划线“_”变量的目的是什么？ 4个答案

Peter Norvig has an essay describing a program to solve sudoku puzzles, even the hardest ones, by combining deterministic logical operations and smart traversal of the possible solutions. The latter is done recursively; here's that function (source):

Peter Norvig通过结合确定性逻辑运算和可能解决方案的智能遍历，撰写了一篇描述解决数独难题的程序的文章，即使是最困难的难题。后者是递归完成的;这是该功能（来源）：

def search(values):
    "Using depth-first search and propagation, try all possible values."
    if values is False:
        return False ## Failed earlier
    if all( len( values[s]) == 1 for s in squares): 
        return values ## Solved!
    ## Chose the unfilled square s with the fewest possibilities
    _,s = min( (len( values[s]), s) 
                for s in squares 
                if len(values[s]) > 1
            )
    return some( search( assign( values.copy(), s, d)) 
                for d in values[s]
            )

(I've added some spaces, CRs, and tabs for the sake of my eyes; apologies to Dr. Norvig.)

（为了我的眼睛，我添加了一些空格，CR和标签;向Norvig博士道歉。）

Right below the comment there's a line starting with "_,s". That seems to be the unpacked tuple (len(values[s]),s) with the minimal value of s. Is Dr. Norvig using "_" as a variable name just to indicate it's a "don't care" result, or is something else going on? Are there times when "_" is recommended as a variable name? In interactive mode, "_" holds the answer of the previous operation; is there a similar function in non-interactive code?

在评论的正下方有一行以“_，s”开头。这似乎是具有s的最小值的解包元组（len（values [s]），s）。 Norvig博士是否使用“_”作为变量名称只是为了表明它是“不关心”的结果，还是其他事情正在发生？是否有时候建议将“_”作为变量名？在交互模式下，“_”代表前一个操作的答案;在非交互式代码中是否有类似的功能？

Update

Thanks for the good answers. I guess The Answer goes to Alex Martelli for "value added"; he points out that the "_, vbl_of_interest" idiom is often a side effect of the DSU idiom, which itself has been made largely unnecessary.

谢谢你的好答案。我猜答案是Alex Martelli的“增值”;他指出，“_，vbl_of_interest”成语通常是DSU成语的副作用，而这本身就很大程度上是不必要的。

3 个解决方案

#1

Yep, _ is a traditional name for "don't care" (which unfortunately *es with its use in I18N, but that's a separate issue;-). BTW, in today's Python, instead of:

是的，_是“不关心”的传统名称（不幸的是它在I18N中的使用发生冲突，但这是一个单独的问题;-)。 BTW，在今天的Python中，而不是：

_,s = min( (len( values[s]), s) 
            for s in squares 
            if len(values[s]) > 1
        )

you might code

你可以编码

s = min((s for s in squares if len(values[s])>1), 
        key=lambda s: len(values[s]))

(not sure what release of Python Peter was writing for, but the idiom he's using is an example of "decorate-sort-undecorate" [[DSU]] except with min instead of sort, and in today's Python the key= optional parameter is generally the best way to do DSU;-).

（不确定Python正在编写什么版本的Python，但他使用的成语是“decorate-sort-undecorate”[[DSU]]的例子，除了min而不是sort，在今天的Python中key = optional参数是一般来说做DSU的最好方法;-)。

#2

You are correct. In non-interactive mode _ has no special meaning. Indeed, Norvig just wants to convey that he doesn't care about the value of that variable.

你是对的。在非交互模式中_没有特殊含义。实际上，Norvig只是想表达他并不关心那个变量的价值。

_{Offtopic: That article by Norvig is very nice. A recommended read.}

Offtopic：Norvig的那篇文章非常好。推荐阅读。

#3

Your interpretation is correct. Outside of the special meaning in interactive mode _ is just used as a "don't care" variable name, especially in unpacking.

你的解释是正确的。在交互模式_中的特殊含义之外_仅用作“不关心”变量名，特别是在解包时。

#1