Flatten a List Python

Question

Consider a List that has a several nested list inside it.

['String example 1',
'String example 2',
['String example 3'],
['String example 4', 'String example 5'],
'String example 6']

The output should be

['String example 1',
'String example 2',
'String example 3',
'String example 4',
'String example 5',
'String example 6',]

I've use this code but the problem it also flatten a non nested string

flat_list = [num for sublist in text for num in sublist]

one of the easiest way is to use - `sum(sublist,[])` – Vaebhav Apr 26 '22 at 03:46 — Vaebhav, Apr 26 '22 at 03:46
@Vaebhav It wont work cuz there are strings – Abhyuday Vaish Apr 26 '22 at 04:01 — Abhyuday Vaish, Apr 26 '22 at 04:01
Before asking a question, please search for existing Q&As. – kaya3 Apr 26 '22 at 04:36 — kaya3, Apr 26 '22 at 04:36

Abhyuday Vaish · Answer 1 · 2022-04-26T03:52:24.883

Here's how you can do it. Check if the datatype of the item is a list or not. If it's not then append it directly. But if it's a list then append by looping through its items. :

flat_list = []
for sublist in text:
    if type(sublist) == list:
        for num in sublist:
            flat_list.append(num)
        
    else:
        flat_list.append(sublist)

Output:

['String example 1', 'String example 2', 'String example 3', 'String example 4', 'String example 5', 'String example 6']

score 0 · Answer 2 · answered Apr 26 '22 at 03:18

You can use isinstance(..., list) to check whether an item is a list, and then flatten the sublist:

lst = ['String example 1',
'String example 2',
['String example 3'],
['String example 4', 'String example 5'],
'String example 6']

def flatten(lst):
    for x in lst:
        if isinstance(x, list):
            yield from flatten(x)
        else:
            yield x

output = list(flatten(lst))
print(output)
# ['String example 1', 'String example 2', 'String example 3', 'String example 4', 'String example 5', 'String example 6']

score 0 · Answer 3 · answered Apr 26 '22 at 04:32

I like @j1-lee 's answer, but using yield function (for recursion!) might be a bit confusing. Here is a more beginner-friendly version:

def flatten(lst):
    new_lst = []
    for x in lst:
        if isinstance(x, list):
            new_lst += flatten(x)
        else:
            new_lst.append(x)
    return new_lst

Flatten a List Python

3 Answers3