how to flatten/penetrate an arbitrarily structured nested list in Python?

Question

Possible Duplicate:
Flatten (an irregular) list of lists in Python

for instance, from:

[[1,2,3],'a',[[4],[5,6],7]]

we want to flatten/penetrate the structure and get all the elements at bottom listed in a line:

[1,2,3,'a',4,5,6,7]

score 2 · Accepted Answer · answered Jun 07 '12 at 23:23

2

# a recursive function to flatten arbitrary nested lists into one simple 1D list
def flatten(inlist,outlist):   
    for e in inlist:
        if isinstance(e,list) :
            flatten(e,outlist)
        else:
            outlist.append(e)

it's a practice of recursive function, :). the "outlist" here serves as a reference for return-list.

there must be better structures...

;) for example:

answered Jun 07 '12 at 23:23

Matt

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This is a case where recursion works well. – Akavall Jun 07 '12 at 23:29
Btw, You should've included the "answer" as part of the question. – Akavall Jun 07 '12 at 23:30
4

@Akvall: no, putting this as an answer is correct, since it answers the question! – Eric Jun 07 '12 at 23:35
@Eric, maybe it OK here, but typically people post their own solution in the question part, since they are looking for improvement. – Akavall Jun 07 '12 at 23:39
@Akvall: Is the asker looking for improvement? – Eric Jun 07 '12 at 23:41
1

@Eric, I thought so..., but perhaps the winky smiley that I missed tells a different story. – Akavall Jun 07 '12 at 23:50
is this an efficient way to flatten nested list? – hugemeow Apr 24 '15 at 06:58

score 0 · Answer 2 · answered Jun 08 '12 at 00:53

0

For your benefit, here is an implementation that modifies the list in-place:

def flatten_in_place(seq):
    if isinstance(seq, list):
        for index, item in reversed(list(enumerate(seq))):
            if isinstance(item, list):
                seq[index: index + 1] = fil(item)
        return seq
    else:
        return [seq]

Usage:

>>> l = [[1, 2], [3, 4], 5, 6, [7, [8, 9]]]
>>> flatten_in_place(l)
>>> l
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Interestingly enough, the timeit results (t = 1,000,000) show this to be slightly faster than your implementation:

flatten_in_place time: 4.88
flatten time: 5.16

I'm not sure why that is exactly. Any ideas?

answered Jun 08 '12 at 00:53

Joel Cornett

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I get an error on `fil` not being defined. – Chris Jun 18 '17 at 17:45
It's been quite a while since I've written this, but I believe `fil` is meant to be `flatten_in_place`. Will update this answer when I've had a chance to test it – Joel Cornett Jun 18 '17 at 18:01
Or, if you get a chance to test it, feel free to update it ;) – Joel Cornett Jun 18 '17 at 18:01

how to flatten/penetrate an arbitrarily structured nested list in Python?

2 Answers2