pygame精灵不会在每次精灵更新时自动移动

import pygame
from pygame.locals import *

# 创建精灵类
class MySprite(pygame.sprite.Sprite):
    def __init__(self):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.Surface((50, 50))
        self.image.fill((255, 0, 0))
        self.rect = self.image.get_rect()
        self.rect.topleft = (100, 100)
        self.speed = 5

    # 更新精灵的位置
    def update(self):
        self.rect.x += self.speed

# 初始化pygame
pygame.init()

# 创建游戏窗口
screen = pygame.display.set_mode((800, 600))
pygame.display.set_caption("Sprite Movement Demo")

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

# 创建精灵对象
sprite = MySprite()
sprites.add(sprite)

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

    # 更新精灵组
    sprites.update()

    # 绘制精灵
    screen.fill((255, 255, 255))
    sprites.draw(screen)

    pygame.display.flip()

# 退出游戏
pygame.quit()