Missing list flatten function in Python

flatten.py

def flatten(items: list, result: list = None) -> list:
    if result is None:
        result = []
    for item in items:
        if isinstance(item, list):
            flatten(item, result)
        else:
            result.append(item)
    return result

test_flatten.py

from django.test import TestCase

from .flatten import flatten


class FlattenTestCase(TestCase):
    def test_flatten_items(self):
        given = flatten([1, 2, 4, 5])
        expected = [1, 2, 4, 5]
        self.assertEqual(expected, given)

    def test_flatten_lists(self):
        given = flatten([[1, 2], [4, 5]])
        print("GIVEN", given)
        expected = [1, 2, 4, 5]
        self.assertEqual(expected, given)

    def test_flatten_mixed_values(self):
        given = flatten([[1, 2], 12, [4, 5]])
        expected = [1, 2, 12, 4, 5]
        self.assertEqual(expected, given)