如何让一个物体在pygame中自动上下移动？

import pygame
pygame.init()

window_width = 800
window_height = 600
window = pygame.display.set_mode((window_width, window_height))
pygame.display.set_caption("自动上下移动物体")

object_x = 400
object_y = 300
object_speed = 5

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

    # 清空窗口
    window.fill((255, 255, 255))

    # 更新物体位置
    object_y += object_speed

    # 边界检测，使物体在窗口边界上下往返移动
    if object_y <= 0 or object_y >= window_height:
        object_speed *= -1

    # 绘制物体
    pygame.draw.rect(window, (0, 0, 255), (object_x, object_y, 50, 50))

    # 刷新窗口
    pygame.display.flip()

# 退出游戏
pygame.quit()