Example of bauncing random balls with Pygame.

bouncing.py

import random
import pygame


WIDTH, HEIGHT = 800, 600

class Ball(pygame.sprite.Sprite):
    def __init__(self, diameter, color=None):
        super().__init__()
        self.diameter = diameter
        self.color = color
        if color is None:
            color = tuple(random.randrange(0, 256) for _ in range(3))
        self.rect = pygame.Rect(random.randrange(0, WIDTH), random.randrange(0, HEIGHT), diameter, diameter)
        self.image = pygame.surface.Surface((diameter, diameter), pygame.SRCALPHA)
        pygame.draw.ellipse(self.image, color, (0, 0, diameter, diameter))
        phong = int(2 * diameter / 3 - diameter / 7)
        pygame.draw.ellipse(self.image, (255,255,255), (phong, phong, diameter// 5, diameter // 5))

        self.vx = random.randrange(0, 15,3)
        self.vy = random.randrange(0, 15, 3)

    def update(self):
        self.rect.left += self.vx
        self.rect.top += self.vy

        if self.rect.left + self.diameter > WIDTH or self.rect.left < 0:
            self.rect.left -= self.vx * 2
            self.vx = -self.vx

        if self.rect.top + self.diameter > HEIGHT or self.rect.top < 0:
            self.rect.top -= self.vy * 2
            self.vy = -self.vy

def init():
    global screen
    screen = pygame.display.set_mode((WIDTH, HEIGHT))

def main():
    balls = pygame.sprite.OrderedUpdates()
    while True:
        pygame.event.pump()
        keys = pygame.key.get_pressed()
        if keys[pygame.K_ESCAPE]:
            break
        if keys[pygame.K_SPACE]:
            balls.add(Ball(random.randrange(12, 64, 12)))
        balls.update()
        screen.fill((0, 0, 0))
        balls.draw(screen)
        pygame.display.flip()
        pygame.time.delay(30)


try:
    init()
    main()
finally:
    pygame.quit()