python飞船游戏（三）

#ship.py

class Ship(Sprite):
    
    def __init__(self,ai_settings,screen):
        """初始化飞船并设置其初始位置"""
        super(Ship,self).__init__()
        self.screen = screen
        self.ai_settings = ai_settings
         
        #加载飞船图像并获取其外接矩形
        self.image = pygame.image.load('images/ship.png')
        self.rect = self.image.get_rect()
        self.screen_rect = screen.get_rect()
        
        #将每艘新飞船放在屏幕底部中央
        self.rect.centerx = self.screen_rect.centerx
        self.rect.bottom = self.screen_rect.bottom
        
         #在飞船的属性center中存储小数值
        self.center = float(self.rect.centerx)
        
        #移动标志
        self.moving_right = False
        self.moving_left = False
    
    def update(self):
        """根据移动标志调整飞船的位置"""
        #更新飞船的center值而不是rect
        if self.moving_right and self.rect.right < self.screen_rect.right:
            self.center += self.ai_settings.ship_speed_factor
            
        if self.moving_left and self.rect.left > 0:
            
            se