game of life finally working

import random
import pygame as pg
import copy

pg.init()
screen = pg.display.set_mode((300, 300))
clock = pg.time.Clock()

grid_size = 100
old_grid = [[0 for x in range(grid_size)] for y in range(grid_size)]

for i in range(0, grid_size):
    for j in range(0, grid_size):
        old_grid[i][j] = random.randrange(0, 2)
new_grid = copy.deepcopy(old_grid)

def cell_block(x, y, state):
    cell_size = 3
    pg.draw.rect(screen, (255, 255, 255) if state == 1 else (0, 0, 0), (x * cell_size, y * cell_size, cell_size, cell_size))

def wrapindex(i):
    return (i % grid_size + grid_size) % grid_size

running = True
while running:
    screen.fill((0, 0, 0))
    clock.tick(60)
    for event in pg.event.get():
        if event.type == pg.QUIT:
            running = False
    for x in range(len(old_grid)):
        for y in range(len(old_grid[x])):
            neighbors = old_grid[wrapindex(x - 1)][wrapindex(y - 1)] + old_grid[wrapindex(x)][wrapindex(y - 1)] + old_grid[wrapindex(x + 1)][wrapindex(y - 1)] + \
                        old_grid[wrapindex(x - 1)][wrapindex(y)]     +                        0                       + old_grid[wrapindex(x + 1)][wrapindex(y)] + \
                        old_grid[wrapindex(x - 1)][wrapindex(y + 1)] + old_grid[wrapindex(x)][wrapindex(y + 1)] + old_grid[wrapindex(x + 1)][wrapindex(y + 1)]

            if neighbors == 3:
                new_grid[x][y] = 1
            elif neighbors > 3:
                new_grid[x][y] = 0
            elif neighbors < 2:
                new_grid[x][y] = 0

            cell_block(x, y, new_grid[x][y])

    pg.display.flip()
    old_grid = copy.deepcopy(new_grid)