避免在pygame中让我的角色从屏幕上掉下来

import pygame
import sys

# 初始化pygame
pygame.init()

# 设置屏幕大小
screen_width = 800
screen_height = 600
screen = pygame.display.set_mode((screen_width, screen_height))
pygame.display.set_caption("Avoid Falling")

# 设置角色初始位置和速度
player_width = 50
player_height = 50
player_x = screen_width // 2 - player_width // 2
player_y = screen_height - player_height
player_speed = 5

# 创建角色矩形对象
player_rect = pygame.Rect(player_x, player_y, player_width, player_height)

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

    # 处理角色移动
    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        player_rect.x -= player_speed
    if keys[pygame.K_RIGHT]:
        player_rect.x += player_speed
    if keys[pygame.K_UP]:
        player_rect.y -= player_speed
    if keys[pygame.K_DOWN]:
        player_rect.y += player_speed

    # 边界检测
    if player_rect.left < 0:
        player_rect.left = 0
    if player_rect.right > screen_width:
        player_rect.right = screen_width
    if player_rect.top < 0:
        player_rect.top = 0
    if player_rect.bottom > screen_height:
        player_rect.bottom = screen_height

    # 绘制角色
    screen.fill((255, 255, 255))
    pygame.draw.rect(screen, (0, 0, 255), player_rect)
    pygame.display.flip()