def test_flatten_primitive() -> None:
flat = flatten(1)
assert flat == [1]
def test_flatten_depth_first() -> None:
flat = flatten([[1, 2], [3, [4, 5]]])
assert flat == [1, 2, 3, 4, 5]
def test_flatten_nested_list() -> None:
flat = flatten([[1, 2], ['a', -1]])
assert flat == [1, 2, 'a', -1]
def test_flatten_obj() -> None:
obj = object()
flat = flatten(obj)
assert flat[0] is obj
assert flat == [obj]
def test_flatten_list() -> None:
lst = [1, 2, 3]
flat = flatten(lst)
assert flat == lst
def test_flatten_str() -> None:
flat = flatten('foo')
assert flat == ['foo']