import pygame
import sys

width=640
height=480
x = 163
y = 120
dx = 1
dy = 1

black = (0,0,0)
white = (255,255,255)
yellow = (255,255,0)

pygame.init()
pygame.display.set_caption("JMJS Python class: bounce_j1")
screen = pygame.display.set_mode((640,480))

clock = pygame.time.Clock()
while 1:
    clock.tick(30)

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

    x += dx
    y += dy

    if x < 0 or x > width:  
        dx = -dx

    if y < 0 or y > height:
        dy = -dy

    screen.fill(black)

    pygame.draw.circle(screen, white, (x,y), 8)

    pygame.display.flip()