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;
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.
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)