Pygame script to generate "crawling" effect

crawler.py

import pygame, random, sys, time

def main():
	width = 640
	height = 480
	x = width / 2
	y = height / 2
	shade = 0

	pygame.init()
	screen = pygame.display.set_mode((width, height))

	while 1:
		for event in pygame.event.get():
			if event.type == pygame.QUIT: 
				sys.exit()

		direction = random.randint(1, 4)

		if direction == 1: x = (x - 1) % width
		elif direction == 2: x = (x + 1) % width
		elif direction == 3: y = (y - 1) % height
		elif direction == 4: y = (y + 1) % height

		shade = (shade + 1) % 255

		screen.set_at((x, y), (shade, shade, shade))

		if shade == 0: # throttle the screen updates
			pygame.display.flip()
			time.sleep(0.001)

if __name__ == "__main__":
	main()