Python pygame精灵在到达屏幕底部后不再向上移动

import pygame
from pygame.locals import *

# 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))

# 定义精灵类
class Sprite(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.centerx = screen_width // 2
        self.rect.bottom = 0
        self.speed = 5

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

# 创建精灵对象
sprite = Sprite()

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

# 游戏主循环
running = True
while running:
    # 处理事件
    for event in pygame.event.get():
        if event.type == QUIT:
            running = False

    # 更新精灵
    all_sprites.update()

    # 绘制背景
    screen.fill((255, 255, 255))

    # 绘制精灵
    all_sprites.draw(screen)

    # 更新屏幕
    pygame.display.flip()

# 退出游戏
pygame.quit()