Hi Mark,

I think this has already been implemented.

For Iterables (list included), there is `more_itertools.collapse`, which you can use to achieve examples in your code as `list(more_itertools.collapse(list))`. See: https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.collapse

If you don’t mind intermediate conversions:
a) For well behaved arrays, such as `numpy.ndarray`, you can do `np.ravel(list).tolist()` or there is a bit more functional `torch.flatten` (I think `numpy` could add this functionality too - it is quite useful).
b) For ragged arrays, there is `awkward.flatten`.

Given that your need is not related to `numpy.ndarray` objects, I don’t think `numpy` is the right place for it. I would alternatively suggest exploring `more_itertools` and if something is missing, issue `Iterator` related proposals over there.


On 31 Dec 2024, at 06:37, Mark via NumPy-Discussion <numpy-discussion@python.org> wrote:

Hello all,

Many people have asked how to flatten a nested list into a one-dimensional list (e.g., see this StackOverflow thread). While flattening a 2D list is relatively straightforward, deeply nested lists can become cumbersome to handle. To address this challenge, I propose adding a built-in list-flattening functionality to NumPy.

 

By adding this feature to NumPy, the library would not only simplify a frequently used task but also enhance its overall usability, making it an even more powerful tool for data manipulation and scientific computing.

 

The code snippet below demonstrates how a nested list can be flattened, enabling conversion into a NumPy array. I believe this would be a valuable addition to the package. See also this issue. 

from collections.abc import Iterable

 

def flatten_list(iterable):

   

    """
    Flatten a (nested) list into a one-dimensional list.

   

    Parameters
    ----------
    iterable : iterable
        The input collection.

   

    Returns
    -------
    flattened_list : list
        A one-dimensional list containing all the elements from the input,
        with any nested structures flattened.

   

    Examples
    --------

   

    Flattening a list containing nested lists:

   

    >>> obj = [[1, 2, 3], [1, 2, 3]]
    >>> flatten_list(obj)
    [1, 2, 3, 1, 2, 3]

   

    Flattening a list with sublists of different lengths:

   

    >>> obj = [1, [7, 4], [8, 1, 5]]
    >>> flatten_list(obj)
    [1, 7, 4, 8, 1, 5]

   

    Flattening a deeply nested list.

   

    >>> obj = [1, [2], [[3]], [[[4]]],]
    >>> flatten_list(obj)
    [1, 2, 3, 4]

   

    Flattening a list with various types of elements:

   

    >>> obj = [1, [2], (3), (4,), {5}, np.array([1,2,3]), range(3), 'Hello']
    >>> flatten_list(obj)
    [1, 2, 3, 4, 5, 1, 2, 3, 0, 1, 2, 'Hello']

   

    """

   

    if not isinstance(iterable, Iterable) or isinstance(iterable, str):
        return [iterable]

   

    def flatten_generator(iterable):

       

        for item in iterable:

           

            if isinstance(item, Iterable) and not isinstance(item, str):
                yield from flatten_generator(item)

           

            else:
                yield item

   

    return list(flatten_generator(iterable))



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