How to expand list comprehension in Python?

Question

I understand that in a normal format of list comprehension like [i for j in k for i in j] This basically can be expanded as:

empty_list = []    
for j in k:
    for i in j:
        empty_list.append(i)

However, I got one question in a recursive function like flatten a nested list with recursive:

def flatten(l:List):
    if isinstance(l, list):
        return [i for j in l for i in flatten(j)]
    else: return [l]

If I just expand this list comprehension, flatten(j) will be in NoneType:

for j in l:
    for i in flatten(j):
        empty_list.append(i)

This will run out as 'NoneType' object is not iterable.

How to expand [i for j in l for i in flatten(j)] in a proper way? Thanks.

Answer 1

This is because in your second example you didn't return a list, you appended to a global. For that to work you would want the following:

def flatten(l):
    if isinstance(l, list):
        empty_list = []
        for j in l:
            for i in flatten(j):
                empty_list.append(i)
        return empty_list
    else:
        return [l]

Realistically, I wouldn't use either approach. You're creating a bunch of throw away containers which is expensive. Instead I would use a generator approach:

def flatten(l):
    if isinstance(l, list):
        for j in l:
            yield from flatten(j)
    else:
        yield l

data = [1, [2, [3]]]
print(list(flatten(data)))

[1, 2, 3]