python list comprehensions; compressing a list of lists?

Question

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.

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] 

But this return a list of lists;

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

What I really want is to get the flattened list;

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

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?

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

Answer 1

You can have nested iterations in a single list comprehension:

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

which is equivalent (at least functionally) to:

filenames = [] for path in dirs:     for filename in os.listdir(path):         filenames.append(filename) 

Answer 2

>>> from functools import reduce >>> listOfLists = [[1, 2],[3, 4, 5], [6]] >>> reduce(list.__add__, listOfLists) [1, 2, 3, 4, 5, 6] 

I'm guessing the itertools solution is more efficient than this, but this feel very pythonic.

In Python 2 it avoids having to import a library just for the sake of a single list operation (since reduce is a built-in).