Pygame中的窗口边界不起作用

import pygame

# 初始化Pygame
pygame.init()

# 设置屏幕尺寸
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))

# 游戏对象类
class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.Surface([50, 50])
        self.image.fill((255, 0, 0))
        self.rect = self.image.get_rect()
        self.rect.x = SCREEN_WIDTH // 2
        self.rect.y = SCREEN_HEIGHT // 2
        self.speed_x = 5
        self.speed_y = 5

    def update(self):
        self.rect.x += self.speed_x
        self.rect.y += self.speed_y
        # 边界检测
        if self.rect.left < 0 or self.rect.right > SCREEN_WIDTH:
            self.speed_x = -self.speed_x
        if self.rect.top < 0 or self.rect.bottom > SCREEN_HEIGHT:
            self.speed_y = -self.speed_y

# 创建玩家对象
player = Player()
all_sprites = pygame.sprite.Group(player)

# 游戏循环
running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # 更新游戏对象
    all_sprites.update()

    # 绘制屏幕
    screen.fill((0, 0, 0))
    all_sprites.draw(screen)
    pygame.display.flip()

pygame.quit()