嵌套列表理解如何工作的解释? [英] Explanation of how nested list comprehension works?

问题描述

我对此毫无疑问:

a = [1,2,3,4]
b = [x for x in a]

我以为是全部，但是后来我发现了这个片段:

I thought that was all, but then I found this snippet:

a = [[1,2],[3,4],[5,6]]
b = [x for xs in a for x in xs]

哪个是b = [1,2,3,4,5,6].问题是我很难理解[x for xs in a for x in xs]中的语法，谁能解释它的工作原理吗?

Which makes b = [1,2,3,4,5,6]. The problem is I'm having troubles to understand the syntax in [x for xs in a for x in xs], Could anyone explain how it works?

推荐答案

啊，难以理解的嵌套"理解.循环以与理解相同的顺序展开.

Ah, the incomprehensible "nested" comprehensions. Loops unroll in the same order as in the comprehension.

[leaf for branch in tree for leaf in branch]

这样有助于思考它.

for branch in tree:
    for leaf in branch:
        yield leaf

PEP202 断言这种语法具有最近的索引变化最快"是正确的人"，尤其是在没有解释为什么的情况下.

The PEP202 asserts this syntax with "the last index varying fastest" is "the Right One", notably without an explanation of why.

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