import pygame, time

pygame.init()
screen = pygame.display.set_mode([601,601])
screen.fill([0,0,0])
pygame.display.flip()

for j in range(0,301,20):
    pygame.draw.line(screen, [100,100,255], [0,300-j], [j,0], 2)
    time.sleep(.1)
    pygame.display.flip()

for j in range(0,301,20):
    pygame.draw.line(screen, [100,100,255], [300+j,0], [600,j], 2)
    time.sleep(.1)
    pygame.display.flip()

for j in range(0,301,20):
    pygame.draw.line(screen, [100,100,255], [600,300+j], [600-j,600], 2)
    time.sleep(.1)
    pygame.display.flip()

for j in range(0,301,20):
    pygame.draw.line(screen, [100,100,255], [300-j,600], [0,600-j], 2)
    time.sleep(.1)
    pygame.display.flip()

raw_input("Press ENTER to close the graphics window")
pygame.quit()