It is not so easy. There was reason why it was not done earlier. scandir() wastes more limited resource than memory -- file descriptors. It should also be properly closed and do not depend on the garbage collector.

Consider the example:

def traverse(path, visit):
    for child in path.iterdir():
        if child.is_dir():
            traverse(path, visit)
        else:
            visit(child)

With your optimization it may fail with OSError: Too many open files.