Partially flatten a list up to a given level

Question

As pointed out in comments, most if not all the answers to this related question fail for lists like:

ls = [1,2,[3,4]]

Moreover, the list could be more deeply nested. How to partially flatten up to a user given level (to infinity by default)

ls2 = [1,[2,3],[4,[5,6]]]

Desired output for ls2:

flatten to level 1:

 [1,2,3,4,[5,6]]

flatten to level 2 (or higher)

 [1,2,3,4,5,6]

See this answer: https://stackoverflow.com/a/4590652/10625520 — Heyran.rs, Mar 01 '19 at 16:59

Thierry Lathuille · Answer 1 · 2019-03-01T17:09:31.290

You could do that recursively:

def flatten(l, level=None):
    if level == 0:
        return l
    flattened = []
    for item in l:
        if isinstance(item, list):
            flattened.extend(flatten(item, level-1 if level is not None else None))
        else:
            flattened.append(item)

    return flattened

ls2 = [1,[2,3],[4,[5,6]]]

print(flatten(ls2, level=1))
# [1, 2, 3, 4, [5, 6]]

print(flatten(ls2, level=2))
# [1, 2, 3, 4, 5, 6]

print(flatten(ls2))
# [1, 2, 3, 4, 5, 6]

score 0 · Answer 2 · answered Mar 01 '19 at 17:01

One way to do it

ls2 = [1,[2,3],[4,[5,6]]]

def make_list_of_list(a):
    return [[i]if not isinstance(i, list) else i for i in a]

def flatten(l):
    return [item for sublist in make_list_of_list(l) for item in sublist]

flatten(ls2) will result in [1, 2, 3, 4, [5, 6]]
flatten(flatten(ls2)) will result in [1, 2, 3, 4, 5, 6]

Partially flatten a list up to a given level

2 Answers2