嘿，我不能在我的pygame程序中使用midbottom功能。每次我运行我的程序时，我都能在左下角看到我的rect对象

import pygame

# 初始化pygame
pygame.init()

# 设置屏幕宽高
screen_width = 800
screen_height = 600
screen = pygame.display.set_mode((screen_width, screen_height))

# 创建矩形对象
rect_width = 100
rect_height = 50
rect = pygame.Rect(0, 0, rect_width, rect_height)

# 将矩形对象位置设置为屏幕正中间底部
rect.midbottom = (screen_width / 2, screen_height)

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

    # 绘制矩形对象
    screen.fill((0, 0, 0))
    pygame.draw.rect(screen, (255, 255, 255), rect)
    pygame.display.flip()

# 退出游戏
pygame.quit()