python 飞船添加子弹射击，代码照着书上来但是打不出子弹按空格键直接弹出错误_python 外星人入侵游戏 from ship import ship 代码错误

alien_invasion.py的代码
import sys
import pygame
from settings import Settings
from ship import Ship
from bullet import  Bullet
class AlienInvasion:
    '''管理游戏资源并创建游戏资源'''
    def __init__(self):
        pygame.init()
        self.settings = Settings()

        self.screen = pygame.display.set_mode((0,0), pygame.FULLSCREEN)
        self.settings.screen_width = self.screen.get_rect().width
        self.settings.screen_height = self.screen.get_rect().height


        pygame.display.set_caption("Alien INVASION")

        self.ship = Ship(self)
        self.bullets = pygame.sprite.Group()


    def run_game(self):
        '''开始游戏的主循环'''
        while True:
            self._check_events()
            self.ship.update()
            self.bullets.update()
            self._update_screen()

            #监视键盘和鼠标事件
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    sys.exit()
            self.screen.fill(self.settings.bg_color)
            self.ship.blitme()
            #让最近绘制的屏幕可见
            pygame.display.flip()
    def _check_events(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()