Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

196 Chapter 8

But that’s not all! Because our if statement is also checking

for the left screen boundary (picx <= 0), when our smiley face looks

like it has hit the left side of the screen, it will change speed to

-speed again. If speed is -5, this will change it to -(-5), or +5. So if

our negative speed variable was causing us to move to the left and

up 5 pixels every frame, once we hit picx <= 0 at the left edge of

the screen, speed = -speed will turn speed back to positive 5 , and the

smiley image will start moving to the right and down again, in the

positive x- and y-directions.

Putting it all Together

Try version 1.0 of our app, SmileyBounce1.py, to see the smiley face

bounce from the upper-left corner of the window to the lower-right

corner and back again, never leaving the drawing screen.

SmileyBounce1.py

import pygame # Setup

pygame.init()

screen = pygame.display.set_mode([600,600])

keep_going = True

pic = pygame.image.load("CrazySmile.bmp")

colorkey = pic.get_at((0,0))

pic.set_colorkey(colorkey)

picx = 0

picy = 0

BLACK = (0,0,0)

timer = pygame.time.Clock()

speed = 5

while keep_going: # Game loop

for event in pygame.event.get():

if event.type == pygame.QUIT:

keep_going = False

picx += speed

picy += speed

if picx <= 0 or picx + pic.get_width() >= 600:

speed = -speed

screen.fill(BLACK)

screen.blit(pic, (picx,picy))

pygame.display.update()

timer.tick(60)

pygame.quit() # Exit