Python “贪吃蛇”游戏，在不断改进中学习pygame编程

import pygame
import sys
import random
 
# 初始化pygame
pygame.init()
 
# 设置窗口大小
screen_width = 600
screen_height = 480
 
# 创建窗口
screen = pygame.display.set_mode((screen_width, screen_height))
 
# 设置窗口标题
pygame.display.set_caption('贪吃蛇游戏')
 
# 定义颜色变量
white = (255, 255, 255)
black = (0, 0, 0)
red = (255, 0, 0)
 
# 定义蛇的节点类
class Snake:
    def __init__(self):
        self.body = []
        self.direction = 'right'
        self.length = 3
        self.score = 0
 
    def move(self):
        # 创建新的头部坐标
        new_head = (self.body[0][0], self.body[0][1])
        if self.direction == 'right':
            new_head[0] += 20
        elif self.direction == 'left':
            new_head[0] -= 20
        elif self.direction == 'up':
            new_head[1] -= 20
        elif self.direction == 'down':
            new_head[1] += 20
        self.body.insert(0, new_head)
 
    def draw(self):
        for point in self.body:
            pygame.draw.rect(screen, white, [point[0], point[1], 20, 20])
            pygame.draw.rect(screen, black, [point[0] + 2, point[1] + 2, 16, 16])
 
# 定义食物类
class Food:
    def __init__(self):
        self.x = random.randint(20, screen_width - 20)
        self.y = random.randint(20, screen_height - 20)
        self.x = self.x - (self.x % 20)
        self.y = self.y - (self.y % 20)
 
    def draw(self):
        pygame.draw.rect(screen, red, [self.x, self.y, 20, 20])
 
# 实例化蛇和食物
snake = Snake()
food = Food()
 
# 游戏循环标志
running = True
 
# 游戏主循环
while running:
    # 设置背景颜色
    screen.fill(black)
 
    # 检查事件
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_RIGHT or event.key == ord('d'):
                snake.direction = 'right'
            elif event.key == pygame.K_LEFT or event.key == ord('a'):
                snake.direction = 'left'
            elif event.key == pygame.K_UP or event.key == ord('w'):
                snake.direction = 'up'
            elif event.key == pygame.K_DOWN or event.key == ord('s'):
                snake.di