def tree(cls):
    yield cls.__name__, 0
    yield from sub_tree(cls)              # <1>


def sub_tree(cls):
    for sub_cls in cls.__subclasses__():
        yield sub_cls.__name__, 1         # <2>


def display(cls):
    for cls_name, level in tree(cls):
        indent = ' ' * 4 * level
        print(f'{indent}{cls_name}')


if __name__ == '__main__':
    display(BaseException)