Python list comprehension returns nested list

Question

I built this little program to simulate 2 libraries I want to compare files with.

The code is this:

import os

path = "C:\Users\\nelson\Desktop\Lib Check"

pc_path = os.path.join(path, "pc")
phone_path = os.path.join(path, "phone")

pc_lib = [filename for path, dirname, filename in os.walk(pc_path)]

print pc_lib

it returns

[['1.txt', '2.txt', '3.txt', '4.txt', '5.txt', '6.txt', '8.txt', '9.txt']]

everything is fine except for the fact that the results are in a nested list. Why?

The only way I can stop this is by using

pc_lib = []
for path, dirname, filename in os.walk(pc_path):
    pc_lib.extend(filename)

just for further reference, [here](http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python) is a neat way of flattening lists in Python. — patrick, May 17 '16 at 15:53

Moses Koledoye · Answer 1 · 2016-05-17T15:47:53.480

1

filename is a list of files (the name you've used is not intuitive), so the results are expected

for root, dirs, files in os.walk('my/home/directory'):
     print(files)
#['close_button.gif', 'close_button_red.gif'], 
#['toolbar.js']

extend unwraps the argument list and appends the resulting elements to the list that made the call

edited May 17 '16 at 15:47

answered May 17 '16 at 15:42

Moses Koledoye

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Dan D. · Answer 2 · 2016-05-17T16:17:33.883

1

If you want a list of filenames, I would suggest:

[os.path.join(path, filename) 
 for path, dirnames, filenames in os.walk(pc_path)
 for filename in filenames]

edited May 17 '16 at 16:17

answered May 17 '16 at 15:46

Dan D.

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15
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This would throw a `NameError`: `filenames` is not defined. `for filename in filenames` should come last – Moses Koledoye May 17 '16 at 15:53

score 0 · Answer 3 · answered May 17 '16 at 15:48

os.walk(path) returns an iterator over tuples of (root, dirs, files).

Where

root is the path of the current directory relative to the argument of os.walk
dirs is a list of all the subdirectories in the current directory
files is a list of all normal files in the current directory

If you want a flat list of all files in a filesystem tree, use:

pc_lib = [
    filename for _, _, files in os.walk(pc_path)
    for filename in files
]

You probably want to retain the absolute path, to get thos use:

pc_lib = [
    os.path.join(root, filename)
    for root, _, files in os.walk(pc_path)
    for filename in files
]

Python list comprehension returns nested list

3 Answers3