import pygame, time, random

pygame.init()
screen = pygame.display.set_mode([601,601])
screen.fill([0,200,0])
pygame.display.flip()

x = 300
y = 300
dx = 3
dy = 4

while True:
    pygame.draw.circle(screen, [0,0,0], [x,y], 30)
    pygame.display.flip()
    time.sleep(.003)
    x = x + dx
    y = y + dy
    if x > 570:
        dx = -3
    if x < 30:
        dx = 3
    if y > 570:
        dy = -4
    if y < 30:
        dy = 4

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