import pygame

BLACK = (0, 0, 0)
WHITE = (200, 200, 200)
WINDOW_HEIGHT = 400
WINDOW_WIDTH = 400


pygame.init()
SCREEN = pygame.display.set_mode((WINDOW_HEIGHT, WINDOW_WIDTH))
CLOCK = pygame.time.Clock()
SCREEN.fill(BLACK)


def draw_grid():
    block_size = 20
    for x in range(WINDOW_WIDTH):
        for y in range(WINDOW_HEIGHT):
            rect = pygame.Rect(x*block_size, y*block_size,
                               block_size, block_size)
            pygame.draw.rect(SCREEN, WHITE, rect, 1)


while True:
    draw_grid()
    pygame.display.flip()
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()