python中外星人入侵游戏’Alien’ object has no attribute ‘draw_bullet’

初学python,按照书上代码敲写,遇到一个问题,怎么也解决不了,希望好心人帮助一下孩子。。 问题: Traceback (most recent call last): File “D:/untitled1/alien_invasion/alien_invasion.py”, line 51, in run_game() File “D:/untitled1/alien_invasion/alien_invasion.py”, line 50, in run_game gf.update_screen(ai_settings, screen, stats, ship, aliens, bu