import pygame

# --- constants --- (UPPER_CASE_NAMES)

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600

FPS = 25

GRAY_1 = (128,128,128)
GRAY_2 = (192,192,192)

# --- main ---

pygame.init()

screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))

# ---

background = pygame.surface.Surface(screen.get_size())

color = GRAY_1
for x in range(0, SCREEN_WIDTH, 40):
    for y in range(0, SCREEN_HEIGHT, 40):
        pygame.draw.rect(background, color, [x, y, 40, 40])
        if color == GRAY_1:
            color = GRAY_2
        else:
            color = GRAY_1

# --- mainloop ---

clock = pygame.time.Clock()

running = True
while running:

    # --- events ---

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_ESCAPE:
                running = False

    # --- draws ---

    screen.blit(background, (0,0))

    pygame.display.flip()

    # --- FPS ---

    ms = clock.tick(FPS)

# --- end ---

pygame.quit()