Snake Tutorial - Part 1

import pygame as pg
from pygame.math import Vector2

WIDTH = 500
FPS = 60
GRID = 20
CELLSIZE = WIDTH / GRID

RIGHT = Vector2(1, 0)
LEFT = Vector2(-1, 0)
UP = Vector2(0, -1)
DOWN = Vector2(0, 1)

class Game:
    def __init__(self):
        pg.init()
        self.screen = pg.display.set_mode((WIDTH, WIDTH))
        self.clock = pg.time.Clock()
        pg.display.set_caption('Little Python')

        self.apple = Apple(self.screen, 3, 5)
        self.snake = Snake(self.screen)
        self.snake_timer = 500
        self.snake_clk = 0

    def draw(self):
        self.screen.fill('light green')
        [pg.draw.rect(self.screen, 'black', (x * CELLSIZE, y * CELLSIZE, CELLSIZE, CELLSIZE), 1) for y in range(GRID) for x in range(GRID)]
        self.apple.draw()
        self.snake.draw()

    def control(self):
        keys = pg.key.get_pressed()
        if keys[pg.K_UP]:
            self.snake.change_direction(UP)
        elif keys[pg.K_DOWN]:
            self.snake.change_direction(DOWN)
        elif keys[pg.K_LEFT]:
            self.snake.change_direction(LEFT)
        elif keys[pg.K_RIGHT]:
            self.snake.change_direction(RIGHT)

    def update(self):
        pg.display.update()
        self.clock.tick(FPS)
        now = pg.time.get_ticks()
        if now - self.snake_clk > self.snake_timer:
            self.snake_clk = now
            self.snake.move()

class Snake:
    def __init__(self, screen):
        self.screen = screen
        self.body = [Vector2(10, 10)]
        self.color = 'blue'
        self.direction = RIGHT

    def _get_segment(self, index):
        rect = pg.Rect(self.body[index].x*CELLSIZE, self.body[index].y*CELLSIZE, CELLSIZE, CELLSIZE)
        return rect

    def draw(self):
        for i in range(len(self.body)):
            rect = self._get_segment(i)
            pg.draw.rect(self.screen, self.color, rect)

    def move(self):
        self.body[0] += self.direction

    def change_direction(self, new_dir):
        if new_dir.x == self.direction.x or new_dir.y == self.direction.y:
            return
        self.direction = new_dir


class Apple:
    def __init__(self, screen, x, y):
        self.screen = screen
        self.x = x
        self.y = y

    def draw(self):
        rect = pg.Rect(self.x * CELLSIZE, self.y * CELLSIZE, CELLSIZE, CELLSIZE)
        pg.draw.rect(self.screen, 'red', rect)


def main():
    game = Game()
    running = True
    while running:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                running = False
        game.control()
        game.draw()
        game.update()
    pg.quit()


if __name__ == '__main__':
    main()