Движение змейки в Python на PyGame

import pygame 
import random
W = 600
H = 700
scr = pygame.display.set_mode((W,H))
pygame.display.set_caption('Snake')
pygame.display.set_icon(pygame.image.load('images/snake.png'))
game_over = False
 
app = 2
size_block = 23
x = 0
y = 0
o = 5
blocks = 20
x_1 = 150
y_1 = 190
white = (255,255,255)
red = (204,0,0)
whblue = (51, 204, 255)
green = (0,170,0)
grey = (153,153,153)
grey1 = (102,102,102)
grey2 = (51,51,51)
flag_d = False
flag_u = False
flag_r = False
flag_l = False
 
 
#определение границ
 
 
def granitsa(first, second):
    q = []
    for i in range(first,second):
        q.append(i)
    return q
 
 
y_gran = granitsa(140, 639)
x_gran = granitsa(50, 549)
 
delay = 500
clock = pygame.time.Clock()
 
 
while not game_over:
    for event in pygame.event.get():
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_UP:
                    x_1 += 0
                    y_1 += -25
            elif event.key == pygame.K_DOWN:
                    x_1 += 0
                    y_1 += 25
            elif event.key == pygame.K_LEFT:
                    x_1 += -25
                    y_1 += 0
            elif event.key == pygame.K_RIGHT:
                    x_1 += 25
                    y_1 += 0
            print(x_1, y_1)
        if x_1 not in x_gran or y_1 not in y_gran:
            pygame.quit()
 
        pygame.draw.polygon(scr, grey, [[0,125],[0,900],[650,900],[650,125]])
        pygame.draw.polygon(scr, grey1, [[0,0],[0,124],[650,124],[650,0]])
        pygame.draw.polygon(scr, grey2, [[48,138],[48,638],[548,638],[548,138]], 2)
        if event.type == pygame.QUIT:
            pygame.quit()
 
        #поле
 
 
        for j in range(blocks):
            x1 = 50
            y1 = 140 + (j * blocks) + j * o
            pygame.draw.rect(scr, white,(x1, y1, size_block, size_block))
            for i in range(blocks):
                x = x1 + (i * size_block) + i * 2
                y = y1
                pygame.draw.rect(scr, white,(x, y, size_block, size_block))
        #змейка
        pygame.draw.rect(scr, red, (x_1, y_1, size_block, size_block))
    clock.tick(30)
 
    pygame.display.update()