Bouncy ball

[Ceil]

I am making a pong game and am putting it together a piece at a time. Right now I am working on a simple bouncy ball. I can make it move, but it wont bounce back. Basically I don't know how to set up walls . Here is my code:

#!/usr/bin/python
import pygame
import sys
from pygame.locals import *

x_vel = 1
y_vel = 1
x = 1
y = 1

pygame.init()
screen = pygame.display.set_mode((600,600),0,32)
screen.blit(screen, (0,0))

ball_img = pygame.image.load('ball.png')
ball_cover = pygame.image.load('blank_ball.png')
clock = pygame.time.Clock() 
ball_rect = ball_img.get_rect()

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

     bounds_rect = pygame.Rect(x,y,0,0)
     ball_rect.clamp_ip(bounds_rect)
     x += x_vel
     y += y_vel
     screen.blit(ball_img, ball_rect)
     pygame.display.update()
     clock.tick(50)
     screen.blit(ball_cover, ball_rect)