```"""
This example shows having multiple balls bouncing around the screen at the
same time. You can hit the space bar to spawn more balls.

Sample Python/Pygame Programs
Simpson College Computer Science
http://simpson.edu/computer-science/
"""

import pygame
import random

# Define some colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

SCREEN_WIDTH = 700
SCREEN_HEIGHT = 500
BALL_SIZE = 25

class Ball:
"""
Class to keep track of a ball's location and vector.
"""
def __init__(self):
self.x = 0
self.y = 0
self.change_x = 0
self.change_y = 0

def make_ball():
"""
Function to make a new, random ball.
"""
ball = Ball()
# Starting position of the ball.
# Take into account the ball size so we don't spawn on the edge.
ball.x = random.randrange(BALL_SIZE, SCREEN_WIDTH - BALL_SIZE)
ball.y = random.randrange(BALL_SIZE, SCREEN_HEIGHT - BALL_SIZE)

# Speed and direction of rectangle
ball.change_x = random.randrange(-2, 3)
ball.change_y = random.randrange(-2, 3)

return ball

def main():
"""
This is our main program.
"""
pygame.init()

# Set the height and width of the screen
size = [SCREEN_WIDTH, SCREEN_HEIGHT]
screen = pygame.display.set_mode(size)

pygame.display.set_caption("Bouncing Balls")

# Loop until the user clicks the close button.
done = False

# Used to manage how fast the screen updates
clock = pygame.time.Clock()

ball_list = []

ball = make_ball()
ball_list.append(ball)

# -------- Main Program Loop -----------
while not done:
# --- Event Processing
for event in pygame.event.get():
if event.type == pygame.QUIT:
done = True
elif event.type == pygame.KEYDOWN:
# Space bar! Spawn a new ball.
if event.key == pygame.K_SPACE:
ball = make_ball()
ball_list.append(ball)

# --- Logic
for ball in ball_list:
# Move the ball's center
ball.x += ball.change_x
ball.y += ball.change_y

# Bounce the ball if needed
if ball.y > SCREEN_HEIGHT - BALL_SIZE or ball.y < BALL_SIZE:
ball.change_y *= -1
if ball.x > SCREEN_WIDTH - BALL_SIZE or ball.x < BALL_SIZE:
ball.change_x *= -1

# --- Drawing
# Set the screen background
screen.fill(BLACK)

# Draw the balls
for ball in ball_list:
pygame.draw.circle(screen, WHITE, [ball.x, ball.y], BALL_SIZE)

# --- Wrap-up
# Limit to 60 frames per second
clock.tick(60)

# Go ahead and update the screen with what we've drawn.
pygame.display.flip()

# Close everything down
pygame.quit()

if __name__ == "__main__":
main()

```
﻿