求助：用pygame写的贪吃蛇游戏，蛇走5步自动关闭了

[弦之弈]

求助大佬，我练习写的贪吃蛇小游戏，运行后蛇只走了5步，游戏页面就自动关闭了，请问是怎么回事？
代码如下：

'''贪吃蛇游戏
规定20为1单位长度
可以使用WSAD或方向键控制方向
空格表示暂停'''

import pygame
import time
import sys
import random
from pygame.locals import *

# 初始化pygame
pygame.init()
fpsClock = pygame.time.Clock()
# 创建pygame显示层
playSurface = pygame.display.set_mode((600, 460))  #设置窗口大小
pygame.display.set_caption('Snake Game')  # 设置窗口名称
# 定义颜色变量
redColor = pygame.Color(255, 0, 0)
blackColor = pygame.Color(0, 0, 0)
whiteColor = pygame.Color(255, 255, 255)
greyColor = pygame.Color(150, 150, 150)


# 定义游戏结束函数
def game_over(playSurface, score):
    game_over_fonts = pygame.font.SysFont('arial.ttf', 54)  # 游戏结束字体和大小
    game_over_surf = game_over_fonts.render('Game Over!', True, greyColor)  # 游戏结束内容及颜色
    game_over_rect = game_over_surf.get_rect()
    game_over_rect.midtop = (300, 100)  # 显示位置
    playSurface.blit(game_over_surf, game_over_rect)
    score_font = pygame.font.SysFont('arial.ttf', 54)  # 得分显示
    score_surf = score_font.render('Score:' + str(score), True, greyColor)
    score_rect = score_surf.get_rect()
    score_rect.midtop = (300, 50)
    playSurface.blit(score_surf, score_rect)
    pygame.display.flip()  # 刷新显示页面
    time.sleep(5)
    pygame.quit()
    sys.exit()


# 设置贪吃蛇和食物
snake_position = [100, 100]  # 初始蛇头位置
snake_segments = [[100, 100]]  # 蛇身 初始为1单位
food_position = [300, 300]  # 食物的初始位置
food_num = 1  # 食物的数目为1
direction = 'right'  # 初始方向向右
change_direction = direction
score = 0  # 初始得分

# 按键控制运动轨迹
for event in pygame.event.get():
    if event.type == QUIT:
        pygame.quit()
        sys.exit()
    elif event.type == KEYDOWN:  # 判断键盘输入
        if event.key == K_RIGHT or event.key == ord('d'):
            change_direction = 'right'
        if event.key == K_LEFT or event.key == ord('a'):
            change_direction = 'left'
        if event.key == K_UP or event.key == ord('w'):
            change_direction = 'up'
        if event.key == K_DOWN or event.key == ord('s'):
            change_direction = 'down'
        if event.key == K_ESCAPE:
            pygame.event.post(pygame.event.Event(QUIT))
    # 判断是否输入了反方向
    if change_direction == 'right' and not direction == 'left':
        direction = change_direction
    if change_direction == 'left' and not direction == 'right':
        direction = change_direction
    if change_direction == 'up' and not direction == 'down':
        direction = change_direction
    if change_direction == 'down' and not direction == 'up':
        direction = change_direction
    # 根据方向移动蛇头坐标
    if direction == 'right':
        snake_position[0] += 20
    if direction == 'left':
        snake_position[0] -= 20
    if direction == 'up':
        snake_position[1] -= 20
    if direction == 'down':
        snake_position[1] += 20
    # 增加蛇的长度
    snake_segments.insert(0, list(snake_position))
    # 判断食物是否被吃掉
    if snake_position[0] == food_position[0] and snake_position[1] == food_position[1]:
        food_num = 0
    else:
        snake_segments.pop()  # 如果没有吃掉食物,最后1单位蛇身提出列表
    # 如果吃掉食物，则重新生成食物
    if food_num == 0:
        x = random.randrange(1, 30)  # 和游戏界面大小相关
        y = random.randrange(1, 30)
        food_position = [int(x*20), int(y*20)]
        food_num = 1
        score += 1
    # 判断是否死亡  包括撞墙和撞自身
    if snake_position[0] > 600 or snake_position[0] < 0:
        game_over(playSurface, score)  # 撞击左右边界，游戏结束
    if snake_position[1] > 460 or snake_position[1] < 0:
        game_over(playSurface, score)  # 撞击上下边界，游戏结束
    for snake_body in snake_segments[1:]:
        if snake_position[0] == snake_body[0] and snake_position[1] == snake_position[1]:
            game_over(playSurface, score)
    # 绘制pygame显示层
    playSurface.fill(blackColor)
    for position in snake_segments:
        pygame.draw.rect(playSurface, whiteColor, Rect(position[0], position[1], 20, 20))
        pygame.draw.rect(playSurface, whiteColor, Rect(food_position[0], food_position[1], 20, 20))
    # 刷新pygame显示层
    pygame.display.flip()
    # 控制游戏速度
    fpsClock.tick(1)

[弦之弈]

刚才又试着运行了一下，只走了3步页面就没了。。。

[chuifengde]

while True:    
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        elif event.type == KEYDOWN:  # 判断键盘输入
            if event.key == K_RIGHT or event.key == ord('d'):
                change_direction = 'right'
            if event.key == K_LEFT or event.key == ord('a'):
                change_direction = 'left'
            if event.key == K_UP or event.key == ord('w'):
                change_direction = 'up'
            if event.key == K_DOWN or event.key == ord('s'):
                change_direction = 'down'
            if event.key == K_ESCAPE:
                pygame.event.post(pygame.event.Event(QUIT))
    # 判断是否输入了反方向
    if change_direction == 'right' and not direction == 'left':
        direction = change_direction
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[测试小白白的学习之路]

为什么按键盘无论是方向键还是wasd都没有反应呢 最后只能撞墙死亡