A Solution for André's puzzle here: https://pastebin.com/C9Zbm3dE

andre_read.py

def iterelements(tree):
    """ Tree is a list, generate the element with the logic of the problem
        element can be a value or a list, when a list recur
    """
    for el in tree:
        if isinstance(el, list):
            yield el  # when a list, iterate the list first, then the child
            yield from iterelements(el)
        else:
            yield el


def get_n_element(pos, tree):
    for i, element in enumerate(iterelements(tree)):
        if i == pos:
            return element


class TestKnownGet:
    def test_simple(self):
        assert get_n_element(0, [10, 20, [[30, 40], 50], 60]) == 10
        assert get_n_element(1, [10, 20, [[30, 40], 50], 60]) == 20

    def test_more(self):
        assert get_n_element(2, [10, 20, [[30, 40], 50], 60]) == [[30, 40], 50]
        assert get_n_element(3, [10, 20, [[30, 40], 50], 60]) == [30, 40]
        assert get_n_element(4, [10, 20, [[30, 40], 50], 60]) == 30
        assert get_n_element(5, [10, 20, [[30, 40], 50], 60]) == 40
        assert get_n_element(6, [10, 20, [[30, 40], 50], 60]) == 50
        assert get_n_element(7, [10, 20, [[30, 40], 50], 60]) == 60

andre_write.py

# change way - pure


def change_n_element_with_pos(pos, tree, newval):
    """ Same logic as the reader, but doesn't use yield
        we update the pos as we go, recurring & letting to the recursion do their job
    """
    new_tree = []
    i = 0
    for element in tree:
        if i == pos:
            new_tree.append(newval)
        else:
            if isinstance(element, list):
                new_child, new_pos = change_n_element_with_pos(pos - i - 1, element, newval)
                new_tree.append(new_child)
                i += new_pos
            else:
                new_tree.append(element)
        i += 1
    return new_tree, i


def change_n_element(pos, tree, new_val):
    new_tree, pos = change_n_element_with_pos(pos, tree, new_val)
    return new_tree


class TestKnownChange:
    def test_change_simple(self):
        assert change_n_element(1, [10, 20, 30], 0) == [10, 0, 30]

    def test_change_folded(self):
        assert change_n_element(3, [10, [20, 30], 40], 0) == [10, [20, 0], 40]

    def test_change(self):
        assert change_n_element(0, [10, 20, [[30, 40], 50], 60], 0) == [0, 20, [[30, 40], 50], 60]
        assert change_n_element(2, [10, 20, [[30, 40], 50], 60], 0) == [10, 20, 0, 60]
        assert change_n_element(5, [10, 20, [[30, 40], 50], 60], 0) == [10, 20, [[30, 0], 50], 60]

    def test_post(self):
        assert change_n_element(7, [10, 20, [[30, 40], 50], 60], [[0]]) == [10, 20, [[30, 40], 50], [[0]]]