Python-嵌套列表理解如何工作的解释？

Question

我对此毫无疑问：

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

我以为是全部，但是后来我发现了以下片段：

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

这使b = [1,2,3,4,5,6]。问题是我很难理解中的语法[x for xs in a for x in xs]，有人可以解释它的工作原理吗？

Answer 1

难以理解的“嵌套”理解。循环以与理解相同的顺序展开。

[leaf for branch in tree for leaf in branch]

这样有助于你进行思考。

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