Discussion on "7 Different Ways to Flatten a List of Lists in Python"

Miguel Brito · 2020-12-12T10:03:02.988Z

Ever wondered how can you flatten, or unnest, a 2D list of lists in Python? In another words, how to turn 2-D lists into 1D: [[1, 2], [3, 4], [5, 6, 7], [8]] -> [1, 2, 3, 4, 5, 6, 7, 8]? [[1, 2], [4, 5], [[[7]]], [[[[8]]]]] -> [1, 2, 3, 4, 5, 6, 7,...

Discussion on "7 Different Ways to Flatten a List of Lists in Python" | Hashnode

Ruud van der Ham

Dec 17, 2020

Here's a different, non recursive, solution for deep flattening:

def flatten(bad):
    good  = []
    while bad:
        e = bad.pop()
        if isinstance(e, list):
            bad.extend(e)
        else:
            good.append(e)
    return good[::-1]

Very likely quite fast as well.

HČ

Hynek Černoch

Dec 18, 2020

This require a copy.deepcopy() to prevent a side effects because the original input parameter is completely broken by this function.

Ruud van der Ham

Dec 18, 2020

Hynek Černoch Good point. It is true that the original list will be destroyed. But if that doen't matter a deep copy is not required.

Ruud van der Ham

Dec 18, 2020

Hynek Černoch Maybe it would be better to use this quite different function as an inline flatten, very much like list.sort() or list.remove(). The return value is then always None and the list will be replaced by the flattened version. That can be implemented like:

def flatten_inplace(lst):
    build  = []
    while lst:
        e = lst.pop()
        if isinstance(e, list):
            lst.extend(e)
        else:
            build.append(e)
    lst.extend(build[::-1])

HEE JAE CHOI

dev

Dec 13, 2020

Great article!!

By the way, is it typo?

def flatten_recursive(lst: List[Any]) -> Iterable[Any]:
    """Flatten a list using recursion."""
    for item in lst:
        if isinstance(item, list):
            yield from flatten(item)  # This line! I think it will be `yield from flatten_recursive(item)`
        else:
            yield item

Miguel Brito

Machine Learning Software Engineer - Hobbyst Technical Writer - Interested in Software Testing, Best Practices, Scalability, and Python.

Dec 13, 2020

Thanks HEE JAE CHOI, and yes, this is a typo!

Thanks for flagging it, I've just fixed it. I renamed it before publishing to make it easier to understand and forgot to double check it in IPython.

This is the correct version:

def flatten_recursive(lst: List[Any]) -> Iterable[Any]:
    """Flatten a list using recursion."""
    for item in lst:
        if isinstance(item, list):
            yield from flatten_recursive(item)
        else:
            yield item

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7 Different Ways to Flatten a List of Lists in Python

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