python飞船游戏（三）_pygame里的飞船运动为什么要self.x=float(self.rect.x)然后又赋值回去不能

#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:
            
            self.center -= self.ai_settings.ship_speed_factor
            
        #根据self.center更新rect对象
        self.rect.centerx = self.center
        
    def center_ship(self):
        """让飞船在屏幕上居中"""
        self.center = self.screen_rect.centerx
        
        
    def blitme(self):
        """在指定位置绘制飞船"""
        self.screen.blit(self.image,self.rect)

#bullet.py

class Bullet(Sprite):
    """一个对飞船发射的子弹进行管理的类"""
    
    def __init__(self,ai_settings,screen,ship):
        """在飞船所处的位置创建一个子弹对象"""
        super().__init__()
        self.screen = screen
        
        #在（0，0）处创建一个表示子弹的矩形，再设置正确的位置
        self.rect = pygame.Rect(0,0,ai_settings.bullet_width,
                                ai_settings.bullet_height)
        self.rect.centerx = ship.rect.centerx
        self.rect.top = ship.rect.top
        
        #存储用小数表示的子弹位置
        self.y = float(self.rect.y)
        
        self.color = ai_settings.bullet_color
        self.speed_factor = ai_settings.bullet_speed_factor
    def update(self):
        """向上移动子弹"""
        #更新表示子弹位置的小数值
        self.y -= self.speed_factor
        #更新表示子弹的rect的位置
        self.rect.y = self.y
        
    def draw_bullet(self):
        """在屏幕上绘制子弹"""
        pygame.draw.rect(self.screen,self.color,self.rect)
        
       

#alien.py

class Alien(Sprite):
    """表示单个外星人的类"""
    
    def __init__(self,ai_settings,screen):
        """初始化外星人并设置其起始位置"""
        super(Alien,self).__init__()
        self.screen = screen
        self.ai_settings = ai_settings
        
        #加载外星人图像 并设置其rect属性
        self.image = pygame.image.load('images/alien.png')
        self.rect = self.image.get_rect()
        
        #每个外星人最初都在屏幕左上角附近
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height
        
        #存储外星人的准确位置
        self.x = float(self.rect.x)
        
    def blitme(self):
        """在指定位置绘制外星"""
        self.screen.blit(self.image,self.rect)
        
    def check_edges(self):
        """如果外星人位于屏幕边缘 返回True"""
        screen_rect = self.screen.get_rect()
        if self.rect.right >= screen_rect.right:
            return True
        elif self.rect.left <= 0:
            return True
    
        
    def update(self):
        """向右移动外星人"""
        self.x += (self.ai_settings.alien_speed_factor *
                   self.ai_settings.fleet_direction)
        self.rect.x = self.x

#gamestats

class GameStats():
    """跟踪游戏的信息"""
    
    def __init__(self,ai_settings):
        """初始化统计信息"""
        self.ai_settings = ai_settings
        self.reset_stats()
        
        #游戏刚启动时处于 非 活动状态
        self.game_active = False
        
        #在任何情况下都不应重置最高得分
        self.high_score = 0
        
    def reset_stats(self):
        """初始化在游戏运行期间可能变化的统计信息"""
        self.ships_left = self.ai_settings.ship_limit
        self.score = 0

        self.level = 1

#button.py

class Button():
    
    def __init__(self,ai_settings,screen,msg):
        """初始化按钮的属性"""
        self.screen = screen
        self.screen_rect = screen.get_rect()
        
        #设置按钮的尺寸和其他属性
        self.width,self.height = 200,50
        self.button_color = (255,220,0)
        self.text_color = (255,255,255)
        self.font = pygame.font.SysFont(None,48)
        
        #创建按钮的rect图像居中
        self.rect = pygame.Rect(0,0,self.width,self.height)
        self.rect.center = self.screen_rect.center
        
        #按钮的标签只需要创建一次
        self.prep_msg(msg)
        
    def prep_msg(self,msg):
            """将msg渲染为图像在按钮居中"""
            self.msg_image = self.font.render(msg,True,self.text_color,
                                            self.button_color)
            self.msg_image_rect = self.msg_image.get_rect()
            self.msg_image_rect.center = self.rect.center
            
    def draw_button(self):
            #绘制一个用颜色填充的按钮还有文本
            self.screen.fill(self.button_color,self.rect)

            self.screen.blit(self.msg_image,self.msg_image_rect