the code

import pygame, sys
from pygame.locals import *
 
# game/window constants
SCREENWIDTH = 640
SCREENHEIGHT = 480
BALLSPEED = 4
 
# game specific variables
pscore = 0
cscore = 0
paused = 1
 
# init the pygame modules
pygame.init()
 
screen = pygame.display.set_mode((SCREENWIDTH,SCREENHEIGHT))
pygame.display.set_caption("Pong clone")
 
clock = pygame.time.Clock()
 
# load all images
img_ball   = pygame.image.load("ball.gif").convert()
img_player = pygame.image.load("player.gif").convert()
 
class Player(object):
    def __init__(self,X,Y):
        self.x = X
        self.y = Y
 
    def input(self,event):
        if event.type == KEYDOWN:
            if event.key == K_p:
                if paused:
                    paused=0
                else:
                    paused=1
 
    def draw(self):
        screen.blit(img_player, (self.x,self.y))
 
class Ball(object):
    def __init__(self,X,Y):
        self.x = X
        self.y = Y
        self.xvel = BALLSPEED
        self.yvel = -BALLSPEED
 
    def reset(self):
        this.x = SCREENWIDTH/2
        this.y = SCREENHEIGHT/2
        paused = 1
 
    def draw(self):
        # bounce the ball if it collides with the upper/lower walls
        if self.y+img_ball.get_height() > SCREENHEIGHT or self.y < 0:
            self.yvel-=self.yvel*2
 
        # if someone scores the ball
        if self.x+img_ball.get_width() > SCREENWIDTH:
            reset()
        elif self.x < 0:
            reset()
 
        if not paused:
            self.x+=self.xvel
            self.y+=self.yvel
 
        screen.blit(img_ball, (self.x,self.y))
 
 
# initialize ball object
ball = Ball(SCREENWIDTH/2, SCREENHEIGHT/2)
player = Player(20, SCREENHEIGHT/2)
 
while 1:
    clock.tick(60)
 
    # loop trough all events
    for event in pygame.event.get():
        if event.type == QUIT:
            sys.exit(0)
 
        player.input(event)
 
    # fill background to black color
    screen.fill((0,0,0))
 
    # draw ball on the screen
    ball.draw()
 
    # update screen buffer
    pygame.display.flip()