3. Kicking things off

3.1. The first lines, and loading modules

First off, you need to start off your game and load up your modules. It's always a good idea to set a few things straight at the top of the main source file, such as the name of the file, what it contains, the license it is under, and any other helpful info you might want to give those will will be looking at it. Then you can load modules, with some error checking so that Python doesn't print out a nasty traceback, which non-programmers won't understand. The code is fairly simple, so I won't bother explaining any of it:

#!/usr/bin/env 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)

3.2. Resource handling functions

In the Line By Line Chimp example, the first code to be written was for loading images and sounds. As these were totally independent of any game logic or game objects, they were written as separate functions, and were written first so that later code could make use of them. I generally put all my code of this nature first, in their own, classless functions; these will, generally speaking, be resource handling functions. You can of course create classes for these, so that you can group them together, and maybe have an object with which you can control all of your resources. As with any good programming environment, it's up to you to develop your own best practice and style.

It's always a good idea to write your own resource handling functions, because although Pygame has methods for opening images and sounds, and other modules will have their methods of opening other resources, those methods can take up more than one line, they can require consistent modification by yourself, and they often don't provide satisfactory error handling. Writing resource handling functions gives you sophisticated, reusable code, and gives you more control over your resources. Take this example of an image loading function:

def load_png(name):
	""" Load image and return image object"""
	fullname = os.path.join('data', name)
	try:
		image = pygame.image.load(fullname)
		if image.get_alpha() is None:
			image = image.convert()
		else:
			image = image.convert_alpha()
	except pygame.error, message:
        	print 'Cannot load image:', fullname
        	raise SystemExit, message
	return image, image.get_rect()

Here we make a more sophisticated image loading function than the one provided by Pygame (image.load). Note that the first line of the function is a documentation string describing what the function does, and what object(s) it returns. The function assumes that all of your images are in a directory called data, and so it takes the filename and creates the full pathname, for example data/ball.png, using the os module to ensure cross-platform compatibility. Then it tries to load the image, and convert any alpha regions so you can achieve transparency, and it returns a more human-readable error if there's a problem. Finally it returns the image object, and its rect.

You can make similar functions for loading any other resources, such as loading sounds. You can also make resource handling classes, to give you more flexibility with more complex resources. For example, you could make a music class, with an __init__ function that loads the sound (perhaps borrowing from a load_sound() function), a function to pause the music, and a function to restart. Another handy resource handling class is for network connections. Functions to open sockets, pass data with suitable security and error checking, close sockets, finger addresses, and other network tasks, can make writing a game with network capabilities relatively painless.

Remember the chief task of these functions/classes is to ensure that by the time you get around to writing game object classes, and the main loop, there's almost nothing left to do. Class inheritance can make these basic classes especially handy. Don't go overboard though; functions which will only be used by one class should be written as part of that class, not as a global function.