ball.py

#!/usr/bin/env python
import pygame
import sys
import random

class Ball(object):
    """ ball class, draws and moves the ball """
    def __init__(self, screen, x, y):
        self.screen = screen
        self.x      = x
        self.y      = y
        self.pos    = (self.x, self.y)
        self.radius = 25

        self.draw_ball()

    def draw_ball(self):
        pygame.draw.circle(self.screen,
                           (255,255,255),
                           (int(self.x), int(self.y)),
                           self.radius)

    def move(self, dx, dy):
        self.x += dx
        self.y += dy
        self.draw_ball()

pygame.init()
pygame.display.set_caption('Moving ball')
screen = pygame.display.set_mode((640, 480))

# get the pygame clock
clock  = pygame.time.Clock()
speedx = 500#random.randint(0,250)
speedy = 500#random.randint(0,250)

# initialize the ball
ball = Ball(screen, 100, 200)

# the orientation of the direction
or_x = 1
or_y = 1
while True:
    # check for closing
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
    # get seconds and change dx, dy
    mils = clock.tick()
    secs = mils / 1000.
    dx   = speedx * secs * or_x
    dy   = speedy * secs * or_y
    # refill the black background in order
    # to "delete" the last ball
    screen.fill((0,0,0))
    # move the ball and redraw it
    ball.move(dx, dy)

    # conditions if the ball hits the wall
    # change the orientations accordingly
    if ball.x > 640 - ball.radius:
        or_x *= -1
    elif ball.x < ball.radius:
        or_x *= -1
    elif ball.y > 480 - ball.radius:
        or_y *= -1
    elif ball.y < ball.radius:
        or_y *= -1

    pygame.display.update()