import pygame
import sys

WIDTH=640
HEIGHT=480

x = 163
y = 120
dx = 10
dy = 10

BLACK = (0,0,0)
WHITE = (255,255,255)

pygame.init()
screen = pygame.display.set_mode((WIDTH,HEIGHT))
clock = pygame.time.Clock()
while True:
    clock.tick(10)

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