pygame中没有出现子弹

import pygame
import random

# 初始化pygame
pygame.init()

# 定义窗口尺寸
screen_width = 800
screen_height = 600

# 创建窗口
screen = pygame.display.set_mode((screen_width, screen_height))
pygame.display.set_caption("My Game")

# 定义子弹类
class Bullet(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        self.image = pygame.Surface((10, 10))
        self.image.fill((255, 0, 0))
        self.rect = self.image.get_rect()
        self.rect.center = (x, y)
        self.speed = 5

    def update(self):
        self.rect.y -= self.speed
        if self.rect.bottom < 0:
            self.kill()

# 创建精灵组
all_sprites = pygame.sprite.Group()

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

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE:
                # 创建子弹对象并添加到精灵组
                bullet = Bullet(random.randint(0, screen_width), screen_height)
                all_sprites.add(bullet)

    # 更新精灵组
    all_sprites.update()

    # 绘制背景
    screen.fill((0, 0, 0))

    # 绘制精灵组中的所有子弹
    all_sprites.draw(screen)

    # 更新屏幕
    pygame.display.flip()

# 退出游戏
pygame.quit()