Python列表解析;压缩列表的列表？ [英] python list comprehensions; compressing a list of lists?

问题描述

人。我试图找到一个问题的最优雅的解决方案，并想知道python是否有内置的任何我想要做的事情。

guys. I'm trying to find the most elegant solution to a problem and wondered if python has anything built-in for what I'm trying to do.

我正在做的是这个。我有一个列表， A ，并且我有一个函数 f ，它接受一个项目并返回一个列表。我可以使用列表理解来转换 A 中的所有内容;

What I'm doing is this. I have a list, A, and I have a function f which takes an item and returns a list. I can use a list comprehension to convert everything in A like so;

[f(a) for a in A]

但是这会返回一个列表列表; p>

But this return a list of lists;

[a1,a2,a3] => [[b11,b12],[b21,b22],[b31,b32]]

真正想得到的是扁平化的清单;

What I really want is to get the flattened list;

[b11,b12,b21,b22,b31,b32]

现在，其他语言都有它;它在函数式编程语言中传统上称为 flatmap ，而.Net将它称为 SelectMany 。 Python有没有类似的东西？有没有一种简洁的方式来将函数映射到列表上并将结果展平？

Now, other languages have it; it's traditionally called flatmap in functional programming languages, and .Net calls it SelectMany. Does python have anything similar? Is there a neat way to map a function over a list and flatten the result?

我试图解决的实际问题是：从目录列表开始，查找所有子目录。所以;

The actual problem I'm trying to solve is this; starting with a list of directories, find all the subdirectories. so;

import os
dirs = ["c:\\usr", "c:\\temp"]
subs = [os.listdir(d) for d in dirs]
print subs

currentliy给了我一个清单列表，但我真的很想要一个清单。

currentliy gives me a list-of-lists, but I really want a list.

推荐答案

您可以在单个列表解析中嵌套迭代：

You can have nested iterations in a single list comprehension:

[filename for path in dirs for filename in os.listdir(path)]

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