我怎样才能阻止我的正方形在pygame中移动？

import pygame
import sys

# 初始化pygame
pygame.init()

# 设置窗口大小和标题
size = width, height = 800, 600
screen = pygame.display.set_mode(size)
pygame.display.set_caption("Square Movement")

# 定义正方形的初始位置和速度
square_size = 50
square_x = (width - square_size) // 2
square_y = (height - square_size) // 2
speed = 5

# 游戏循环
while True:
    # 处理退出事件
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()

    # 监听键盘事件
    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        square_x -= speed
    if keys[pygame.K_RIGHT]:
        square_x += speed
    if keys[pygame.K_UP]:
        square_y -= speed
    if keys[pygame.K_DOWN]:
        square_y += speed

    # 检查是否超出窗口边界
    if square_x < 0:
        square_x = 0
    if square_x > width - square_size:
        square_x = width - square_size
    if square_y < 0:
        square_y = 0
    if square_y > height - square_size:
        square_y = height - square_size

    # 清空屏幕
    screen.fill((255, 255, 255))

    # 绘制正方形
    pygame.draw.rect(screen, (0, 0, 0), (square_x, square_y, square_size, square_size))

    # 更新窗口显示
    pygame.display.flip()