Creating nested lists with a list comprehension

Question

I have the following lists:

path='/my/path/'
l1=[2,0,1]
l2=[['a.txt','b.txt','c.txt'],['d.txt','f.txt','g.txt'],['h.txt','i.txt','j.txt']]

and I wrote a list comprehension to add the full path:

[os.path.join(path, 'list%d'%l1_index, l2_value) for l1_index in l1 for l2_value in l2[l1_index]]

but lost the original nesting.

Here's what I'd like to get:

[ ['/my/path/list2/h.txt','/my/path/list2/i.txt','/my/path/list2/j.txt'], ['/my/path/list0/a.txt','/my/path/list0/b.txt','/my/path/list0/c.txt'], ['/my/path/list1/d.txt','/my/path/list1/f.txt','/my/path/list1/g.txt'] ]

Is there a good reason for using a list comprehension here? It's one of those cases where you end up obscuring the intent of the code. — Aidan Kane
– Aidan Kane, Commented Nov 12, 2014 at 23:45

score 2 · Accepted Answer · 2014-11-12 23:49:16Z

2

Use a nested list comprehension instead of a single one with two for ... in ... clauses:

[[os.path.join(path, 'list%d'%l1_index, l2_value) for l2_value in l2[l1_index]] for l1_index in l1]

If you were to spread this out, it would look like:

[
    [
        os.path.join(path, 'list%d'%l1_index, l2_value)
        for l2_value in l2[l1_index]
    ]
    for l1_index in l1
]

Demo:

>>> import os
>>> path='/my/path/'
>>> l1=[2,0,1]
>>> l2=[['a.txt','b.txt','c.txt'],['d.txt','f.txt','g.txt'],['h.txt','i.txt','j.txt']]
>>> [[os.path.join(path, 'list%d'%l1_index, l2_value) for l2_value in l2[l1_index]] for l1_index in l1]
[['/my/path/list2\\h.txt', '/my/path/list2\\i.txt', '/my/path/list2\\j.txt'], ['/my/path/list0\\a.txt', '/my/path/list0\\b.txt', '/my/path/list0\\c.txt'], ['/my/path/list1\\d.txt', '/my/path/list1\\f.txt', '/my/path/list1\\g.txt']]
>>>

edited Nov 12, 2014 at 23:49

answered Nov 12, 2014 at 23:43

user2555451

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Comments

Aidan Kane · Accepted Answer · 2014-11-12 23:49:19Z

0

Personally I'd avoid the nested list comprehensions in this case. It's not especially clear what's going on.

import os

path = '/my/path'
l1 = [2,0,1]
l2 = [
  ['a.txt','b.txt','c.txt'],
  ['d.txt','f.txt','g.txt'],
  ['h.txt','i.txt','j.txt']
]

pathset = []
for l in l1:
    to_add = l2[l]
    paths = [os.path.join(path, 'list%s' % l, filename) for filename in to_add]
    pathset.append(paths)

answered Nov 12, 2014 at 23:49

Aidan Kane

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