Why is my recursive bubble sort not working?

Question

When I run my recursive bubble sort on the list:

 ["4213", "4201", "4204", "4218", "4205", "Out"]

I get:

['4201', '4204', '4213', '4205', '4218', 'Out']

instead of the correct answer. Can anyone explain why?

def test_sort(list_to_sort):
    length = len(list_to_sort)
    if length == 0:
        return list_to_sort
    for i in range(0, length - 1):
        if list_to_sort[i] > list_to_sort[i + 1]:
            (list_to_sort[i], list_to_sort[i + 1]) = (list_to_sort[i + 1], list_to_sort[i])
    test_sort(list_to_sort[:length - 1])
    return list_to_sort

def main():
    testlist = ["4213", "4201", "4204", "4218", "4205", "Out"]
    print(test_sort(testlist))

Answer 1

You forgot to use the results of:

test_sort(list_to_sort[:length - 1])

You can change it to:

list_to_sort[:length - 1] = test_sort(list_to_sort[:length - 1])

Test Code:

def test_sort(list_to_sort):
    length = len(list_to_sort)
    if length < 2:
        return list_to_sort
    for i in range(0, length - 1):
        if list_to_sort[i] > list_to_sort[i + 1]:
            list_to_sort[i:i + 2] = list_to_sort[i + 1], list_to_sort[i]
    list_to_sort[:length - 1] = test_sort(list_to_sort[:length - 1])
    return list_to_sort


def main():
    testlist = ["4213", "4201", "4204", "4218", "4205", "Out"]
    print(test_sort(testlist))

main()

Results:

['4201', '4204', '4205', '4213', '4218', 'Out']