How to flatten a 2 deep list, return a list without any sublists within

Question

I was working on an algorithm that 'flattens' a list, essentially removing any sublists within a list.

For example, [2, 3, [3, 4]] should become [2, 3, 3, 4]. I wrote the following to do this:

def flatten(l):
    total = []
    for i in l:
       if i == int:
           total.append(i)
       elif i == list:
           for j in i:
               total.append(j):
    return total

This algorithm yields an error, however. If someone could help that would be great. Also, if someone could show a recursive path to solve this problem so that lists of arbitrary 'depth' can be flattened (my current algorithm can only flatten a 2D array)

This algorithm yields an error, however. If someone could help that would be great. Also, if someone could show a recursive path to solve this problem so that lists of arbitrary 'depth' can be flattened (my current algorithm can only flatten a 2D array) — user9873466, Jun 12 '18 at 15:38
here's the answer: https://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python — keda, Jun 12 '18 at 15:40
@keda that's not the answer, since this is not a list of lists. This just *has* some lists in it. — 17slim, Jun 12 '18 at 15:41
Possible duplicate of [Flatten an irregular list of lists](https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists) — tourist, Jun 12 '18 at 15:41
This is a duplicate: https://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python — anishtain4, Jun 12 '18 at 15:44
So the question looks exactly the same as this one: https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists — anishtain4, Jun 12 '18 at 15:51

score 2 · Accepted Answer · answered Jun 12 '18 at 15:42

There are two major issues here - first, you aren't checking the type of the item correctly (you should use isinstace) and second you aren't calling the function recursively:

def flatten(l):
    total = []
    for i in l:
       if isinstance(i, list):
           total.extend(flatten(i))       
       else:
           total.append(i)

    return total

How to flatten a 2 deep list, return a list without any sublists within

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