下划线 _ 作为 Python 中的变量名 [英] Underscore _ as variable name in Python

问题描述

Peter Norvig 有一篇文章描述了一个解决数独谜题的程序，即使是最难的数独谜题，通过组合确定性逻辑操作和可能解决方案的智能遍历.后者是递归完成的；这是该函数(source):

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):

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]
            )

(为了我的眼睛，我添加了一些空格、CR 和制表符；向 Norvig 博士道歉.)

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

在评论的正下方有一行以_,s"开头.这似乎是具有 s 最小值的解包元组 (len(values[s]),s).Norvig 博士使用_"作为变量名只是为了表明这是一个无关紧要"的结果，还是其他原因?是否有时会推荐_"作为变量名?在交互模式下，_"保存着上一个操作的答案；非交互式代码中是否有类似的功能?

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?

感谢您的好回答.我想答案是亚历克斯·马泰利的增值"；他指出，_, vbl_of_interest"习语通常是 DSU 习语的副作用，而 DSU 习语本身在很大程度上是不必要的.

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.

推荐答案

是的，_ 是don't care"的传统名称(不幸的是，这与它在 I18N 中的使用冲突，但这就是一个单独的问题;-).顺便说一句，在今天的 Python 中，而不是:

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

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

你可以编码

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

(不知道 Peter 写的是什么版本的 Python，但他使用的习语是decorate-sort-undecorate"[[DSU]] 的一个例子，除了 min 而不是 sort，在今天的 Python 中，key= 可选参数通常是执行 DSU 的最佳方式；-).

(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;-).

这篇关于下划线 _ 作为 Python 中的变量名的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！