如何让我的精灵在pygame中发射火球(子弹)？

import pygame
import random

# 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))
pygame.display.set_caption("Fireball Shooter")

# 定义颜色
WHITE = (255, 255, 255)
RED = (255, 0, 0)

# 定义火球类
class Fireball(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        self.image = pygame.Surface((10, 10))
        self.image.fill(RED)
        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()

# 定义精灵类
class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.Surface((50, 50))
        self.image.fill(WHITE)
        self.rect = self.image.get_rect()
        self.rect.center = (screen_width // 2, screen_height - 50)

    def update(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_LEFT]:
            self.rect.x -= 5
        if keys[pygame.K_RIGHT]:
            self.rect.x += 5

    def shoot(self):
        fireball = Fireball(self.rect.centerx, self.rect.top)
        all_sprites.add(fireball)
        fireballs.add(fireball)

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

# 创建精灵对象
player = Player()
all_sprites.add(player)

# 游戏主循环
running = True
clock = pygame.time.Clock()
while running:
    clock.tick(60)

    # 处理事件
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE:
                player.shoot()

    # 更新精灵和火球
    all_sprites.update()

    # 绘制背景
    screen.fill((0, 0, 0))

    # 绘制精灵和火球
    all_sprites.draw(screen)

    # 更新屏幕
    pygame.display.flip()

# 退出游戏
pygame.quit()