keras/src/tree/tree_api.py/flatten
def flatten(structure):
    """Flattens a possibly nested structure into a list.

    In the case of dict instances, the sequence consists of the values,
    sorted by key to ensure deterministic behavior. This is true also for
    `collections.OrderedDict` instances: their sequence order is
    considered. The same convention is followed in `unflatten_as`.
    This correctly unflattens dicts and `OrderedDict` after they have been
    flattened, or vice-versa.

    Dictionaries with non-sortable keys cannot be flattened.

    Examples:

    >>> keras.tree.flatten([[1, 2, 3], [4, [5], [[6]]]])
    [1, 2, 3, 4, 5, 6]
    >>> keras.tree.flatten(None)
    [None]
    >>> keras.tree.flatten(1)
    [1]
    >>> keras.tree.flatten({100: 'world!', 6: 'Hello'})
    ['Hello', 'world!']

    Args:
        structure: An arbitrarily nested structure.

    Returns:
        A list, the flattened version of the input `structure`.
    """
    return tree_impl.flatten(structure)