## Pygame: Putting it all together

So far you've learnt all the basics necessary to build a simple game. You should understand how to create Pygame objects, how Pygame displays objects, how it handles events, and how you can use physics to introduce some motion into your game. Now I'll just show how you can take all those chunks of code and put them together into a working game. What we need first is to let the ball hit the sides of the screen, and for the bat to be able to hit the ball, otherwise there's not going to be much game play involved. We do this using Pygame's collision methods.

## 6.1. Let the ball hit sides

The basics principle behind making it bounce of the sides is easy to grasp. You grab the coordinates of the four corners of the ball, and check to see if they correspond with the x or y coordinate of the edge of the screen. So if the top right and top left corners both have a y coordinate of zero, you know that the ball is currently on the top edge of the screen. We do all this in the update function, after we've worked out the new position of the ball.

```if not self.area.contains(newpos):
tl = not self.area.collidepoint(newpos.topleft)
tr = not self.area.collidepoint(newpos.topright)
bl = not self.area.collidepoint(newpos.bottomleft)
br = not self.area.collidepoint(newpos.bottomright)
if tr and tl or (br and bl):
angle = -angle
if tl and bl:
self.offcourt(player=2)
if tr and br:
self.offcourt(player=1)

self.vector = (angle,z)
```

Here we check to see if the area contains the new position of the ball (it always should, so we needn't have an else clause, though in other circumstances you might want to consider it. We then check if the coordinates for the four corners are colliding with the area's edges, and create objects for each result. If they are, the objects will have a value of 1, or TRUE. If they don't, then the value will be None, or FALSE. We then see if it has hit the top or bottom, and if it has we change the ball's direction. Handily, using radians we can do this by simply reversing its positive/negative value. We also check to see if the ball has gone off the sides, and if it has we call the offcourt function. This, in my game, resets the ball, adds 1 point to the score of the player specified when calling the function, and displays the new score.

Finally, we recompile the vector based on the new angle. And that is it. The ball will now merrily bounce off the walls and go offcourt with good grace.

## 6.2. Let the ball hit bats

Making the ball hit the bats is very similar to making it hit the sides of the screen. We still use the collide method, but this time we check to see if the rectangles for the ball and either bat collide. In this code I've also put in some extra code to avoid various glitches. You'll find that you'll have to put all sorts of extra code in to avoid glitches and bugs, so it's good to get used to seeing it.

```else:
# Deflate the rectangles so you can't catch a ball behind the bat
player1.rect.inflate(-3, -3)
player2.rect.inflate(-3, -3)

# Do ball and bat collide?
# Note I put in an odd rule that sets self.hit to 1 when they collide, and unsets it in the next
# iteration. this is to stop odd ball behaviour where it finds a collision *inside* the
# bat, the ball reverses, and is still inside the bat, so bounces around inside.
# This way, the ball can always escape and bounce away cleanly
if self.rect.colliderect(player1.rect) == 1 and not self.hit:
angle = math.pi - angle
self.hit = not self.hit
elif self.rect.colliderect(player2.rect) == 1 and not self.hit:
angle = math.pi - angle
self.hit = not self.hit
elif self.hit:
self.hit = not self.hit
self.vector = (angle,z)
```

We start this section with an else statement, because this carries on from the previous chunk of code to check if the ball hits the sides. It makes sense that if it doesn't hit the sides, it might hit a bat, so we carry on the conditional statement. The first glitch to fix is to shrink the players' rectangles by 3 pixels in both dimensions, to stop the bat catching a ball that goes behind them (if you imagine you just move the bat so that as the ball travels behind it, the rectangles overlap, and so normally the ball would then have been "hit" - this prevents that).

Next we check if the rectangles collide, with one more glitch fix. Notice that I've commented on these odd bits of code - it's always good to explain bits of code that are abnormal, both for others who look at your code, and so you understand it when you come back to it. The without the fix, the ball might hit a corner of the bat, change direction, and one frame later still find itself inside the bat. Then it would again think it has been hit, and change its direction. This can happen several times, making the ball's motion completely unrealistic. So we have a variable, self.hit, which we set to TRUE when it has been hit, and FALSE one frame later. When we check if the rectangles have collided, we also check if self.hit is TRUE/FALSE, to stop internal bouncing.

The important code here is pretty easy to understand. All rectangles have a colliderect function, into which you feed the rectangle of another object, which returns 1 (TRUE) if the rectangles do overlap, and 0 (FALSE) if not. If they do, we can change the direction by subtracting the current angle from pi (again, a handy trick you can do with radians, which will adjust the angle by 90 degrees and send it off in the right direction; you might find at this point that a thorough understanding of radians is in order!). Just to finish the glitch checking, we switch self.hit back to FALSE if it's the frame after they were hit.

We also then recompile the vector. You would of course want to remove the same line in the previous chunk of code, so that you only do this once after the if-else conditional statement. And that's it! The combined code will now allow the ball to hit sides and bats.

## 6.3. The Finished product

The final product, with all the bits of code thrown together, as well as some other bits ofcode to glue it all together, will look like this:

```#!/usr/bin/python
#
# Tom's Pong
# A simple pong game with realistic physics and AI
# http://www.tomchance.uklinux.net/projects/pong.shtml
#
# Released under the GNU General Public License

VERSION = "0.4"

try:
import sys
import random
import math
import os
import getopt
import pygame
from socket import *
from pygame.locals import *
except ImportError, err:
print "couldn't load module. %s" % (err)
sys.exit(2)

""" Load image and return image object"""
fullname = os.path.join('data', name)
try:
if image.get_alpha() is None:
image = image.convert()
else:
image = image.convert_alpha()
except pygame.error, message:
raise SystemExit, message
return image, image.get_rect()

class Ball(pygame.sprite.Sprite):
"""A ball that will move across the screen
Returns: ball object
Functions: update, calcnewpos
Attributes: area, vector"""

def __init__(self, (xy), vector):
pygame.sprite.Sprite.__init__(self)
screen = pygame.display.get_surface()
self.area = screen.get_rect()
self.vector = vector
self.hit = 0

def update(self):
newpos = self.calcnewpos(self.rect,self.vector)
self.rect = newpos
(angle,z) = self.vector

if not self.area.contains(newpos):
tl = not self.area.collidepoint(newpos.topleft)
tr = not self.area.collidepoint(newpos.topright)
bl = not self.area.collidepoint(newpos.bottomleft)
br = not self.area.collidepoint(newpos.bottomright)
if tr and tl or (br and bl):
angle = -angle
if tl and bl:
#self.offcourt()
angle = math.pi - angle
if tr and br:
angle = math.pi - angle
#self.offcourt()
else:
# Deflate the rectangles so you can't catch a ball behind the bat
player1.rect.inflate(-3, -3)
player2.rect.inflate(-3, -3)

# Do ball and bat collide?
# Note I put in an odd rule that sets self.hit to 1 when they collide, and unsets it in the next
# iteration. this is to stop odd ball behaviour where it finds a collision *inside* the
# bat, the ball reverses, and is still inside the bat, so bounces around inside.
# This way, the ball can always escape and bounce away cleanly
if self.rect.colliderect(player1.rect) == 1 and not self.hit:
angle = math.pi - angle
self.hit = not self.hit
elif self.rect.colliderect(player2.rect) == 1 and not self.hit:
angle = math.pi - angle
self.hit = not self.hit
elif self.hit:
self.hit = not self.hit
self.vector = (angle,z)

def calcnewpos(self,rect,vector):
(angle,z) = vector
(dx,dy) = (z*math.cos(angle),z*math.sin(angle))
return rect.move(dx,dy)

class Bat(pygame.sprite.Sprite):
"""Movable tennis 'bat' with which one hits the ball
Returns: bat object
Functions: reinit, update, moveup, movedown
Attributes: which, speed"""

def __init__(self, side):
pygame.sprite.Sprite.__init__(self)
screen = pygame.display.get_surface()
self.area = screen.get_rect()
self.side = side
self.speed = 10
self.state = "still"
self.reinit()

def reinit(self):
self.state = "still"
self.movepos = [0,0]
if self.side == "left":
self.rect.midleft = self.area.midleft
elif self.side == "right":
self.rect.midright = self.area.midright

def update(self):
newpos = self.rect.move(self.movepos)
if self.area.contains(newpos):
self.rect = newpos
pygame.event.pump()

def moveup(self):
self.movepos = self.movepos - (self.speed)
self.state = "moveup"

def movedown(self):
self.movepos = self.movepos + (self.speed)
self.state = "movedown"

def main():
# Initialise screen
pygame.init()
screen = pygame.display.set_mode((640, 480))
pygame.display.set_caption('Basic Pong')

# Fill background
background = pygame.Surface(screen.get_size())
background = background.convert()
background.fill((0, 0, 0))

# Initialise players
global player1
global player2
player1 = Bat("left")
player2 = Bat("right")

# Initialise ball
speed = 13
rand = ((0.1 * (random.randint(5,8))))
ball = Ball((0,0),(0.47,speed))

# Initialise sprites
playersprites = pygame.sprite.RenderPlain((player1, player2))
ballsprite = pygame.sprite.RenderPlain(ball)

# Blit everything to the screen
screen.blit(background, (0, 0))
pygame.display.flip()

# Initialise clock
clock = pygame.time.Clock()

# Event loop
while 1:
# Make sure game doesn't run at more than 60 frames per second
clock.tick(60)

for event in pygame.event.get():
if event.type == QUIT:
return
elif event.type == KEYDOWN:
if event.key == K_a:
player1.moveup()
if event.key == K_z:
player1.movedown()
if event.key == K_UP:
player2.moveup()
if event.key == K_DOWN:
player2.movedown()
elif event.type == KEYUP:
if event.key == K_a or event.key == K_z:
player1.movepos = [0,0]
player1.state = "still"
if event.key == K_UP or event.key == K_DOWN:
player2.movepos = [0,0]
player2.state = "still"

screen.blit(background, ball.rect, ball.rect)
screen.blit(background, player1.rect, player1.rect)
screen.blit(background, player2.rect, player2.rect)
ballsprite.update()
playersprites.update()
ballsprite.draw(screen)
playersprites.draw(screen)
pygame.display.flip()

if __name__ == '__main__': main()
```