Python Success Stories

Introduction

My son Nat is autistic. He likes to look at pictures of familiar places, things, and people. We bought a digital camera so we could take as many pictures of as many ordinary things as we wanted, without having to worry about the cost and bother of film and development. Nat's tastes are difficult to predict, so we might take pictures of a subject certain to please him, only to find that he has no interest in them at all. We wanted to be able to cast a wide net while taking pictures, to be certain to capture the images that would interest him.

Once we had the camera, we took hundreds of pictures, and showed Nat how to look through them on the computer using an off-the-shelf image viewer called ThumbsPlus. Watching Nat flip through pictures, though, I noticed something: once he became familiar with the sequence of the pictures, he was clicking on the pictures as if it mattered where he clicked. The image viewer didn't care: any click on the picture would show the next picture. But if Nat knew that the next picture was of a thing off to the left of the current picture, he would click on the left of the image. If he knew the next picture was through a door at the right of the picture, he would click on the door. He's played a number of kid's adventure games, such as Pajama Sam, and knew about this style of navigation. Even in a program that didn't use that navigation style, he had begun using it as if it did.

So I thought, why not make an image viewer that worked his way? Why not write a program that lets me build picture environments from our life, and lets Nat navigate through them in the style he wanted?

Natworld Screen Shot

One of over 1500 images in Natsworld. The user interface is simply a set of directional cursors (near center) Zoom in

Choosing Python

I had used Python for small scripting projects before, and liked its quick-off-the-blocks feel. It would provide me with a malleable environment in which to experiment with features, to see what Nat would find interesting. Although this was an at-home project to be used only by my son, it shared issues with larger, supposedly more serious projects: unclear requirements, unpredictable customers, and limited developer resources. The most important concern to me in choosing the development tools was my productivity. Since this was "after-hours" work, my time and attention would be stretched thin, and I wanted to be able to focus on the user experience, not build environments, memory management, and the like.

I looked for existing software, and found lots of interesting examples at pygame (www.pygame.org). Pygame provided all the functions I'd need for image manipulation and display management, and there were a few existing projects there that provided useful examples. Pyzzle almost provided what I needed, but wasn't as well-organized internally as I would have liked to be able to quickly experiment with new features.

Implementation

Using the existing examples to help guide my pygame use, I created a new framework for my program. My program, which I named simply "Nat's World", would provide a virtual world for exploration. It would work the way Nat expected it to, providing a simple exploration environment similar to games like Myst; clicking on the left of the screen would turn you left, clicking center would move you forward, and so forth.

At its simplest, the entire environment was simply a set of nodes, each of which had an image to display, and a list of other connected nodes to navigate to. One physical location in the world is represented by a number of nodes, one for each direction to face. These nodes together are called a "spot". Over time, this model was extended to add features to the environment, but this simple model got me off the ground. Python gave me a clean object-oriented basis for building my Node class, and pygame provided the primitives for displaying images, creating cursors, and collecting input.

One of the biggest challenges was deciding among ways to organize the description of the world. On the one hand, I could have created a descriptive language for the structure of the world, and written an interpreter to read the description and build the in-memory structures on which the game would run. This would have been a fairly large effort, and would have required me to formalize all of the structures that could have appeared in a world. It also would have meant a longer delay before I had something that Nat could actually use. On the other hand, I could have simply used Python statements to construct the structures directly. This would have been tedious and error-prone.

As a handy middle ground, I used Python statements to build the world, but made heavy use of utility functions and classes to provide short-cuts for common structures. For example, my images are all stored in directories named by date, and I would have had to enter an image filename for each node. The ImgShortcut class made this simple. Defining the __call__ method allowed me to play with the syntax of Python to get the effect I wanted:

class ImgShortcut:
    def __init__(self, fmt):
        self.fmt = fmt

    def __call__(self, arg):
        return self.fmt % (arg)

nov4 =  ImgShortcut(r'C:\img\vol3\20011104\dscf%04d.jpg')
nov10 = ImgShortcut(r'C:\img\vol3\20011110\dscf%04d.jpg')

>>> print nov4(17)
c:\img\vol3\20011104\dscf0017.jpg

As another example, many spots in the world shared the same four-node structure: one for the north view, one for the west view, and so on. Using Python's keyword argument syntax, I was able to write a function to build this common spot type:

def nesw(name, **data):
    """
    Make four nodes for n, e, w, s from a location.
    Keys:
        images: ni, ei, wi, si.
        destinations: n, e, w, s.
    """
    # (code omitted)

The first argument is the base name for the new nodes (the node names are made by appending ":n", ":w", etc). The remaining arguments are specified by keyword: ni is the image name for the north node, n is the node to the north; wi is the image for the west node, w is the node to the west, and so on. Now I can make a spot with a simple short-hand syntax:

nesw('front43',
    ni = nov4(17),
    ei = nov4(18),   e = 'allerton_hawthorne:e',
    si = nov10(381), s = 'hall:s',
    wi = nov4(16),   w = 'markliesl:w'
)

Because of the keyword syntax, I can omit arguments that don't apply for the particular spot. Now I have an almost declarative syntax that is handy for creating common structures, but without having to write an interpreter for a new mini-language. And if I have an unusual case that needs full Python, it is available to me.

The world and the program to support it grew in parallel. As I came up with new ideas for structures for spots, I would write new utility functions like nesw() to create them easily. I could watch Nat play the game, see what he wanted it to do, and add features quickly. Many of Python's features (interpreted code, object orientation, garbage collection, lists and dictionaries) supported this rapid turn-around.

The Node class was extended and subclassed to create new types of nodes. For example, I added a MenuNode subclass to handle on-screen menus of destinations. This allowed clicking on the car, then choosing where you want to drive. Transitions between nodes were added to make the effect more pleasing. Clicking on the stereo produced a menu of songs to play (via pygame, of course).

Results

At this point, the total size of the source is about 1400 lines of code, plus 1300 lines describing the world, which is currently over 1500 nodes.

When I show Nat's World to friends, they always talk about how great it would be if I could make something like it possible for other people. I've thought about writing a WorldBuilder application. It would let a non-technical person use a GUI to browse images, and connect them into a world of nodes. It would be a nice project to build, and an even nicer one to have since I could use it myself to extend Nat's World. I don't know if I'll ever have the time and focus to build such a thing in my spare time, but if I did, I know Python would be the tool for the job.

About the Author

Ned Batchelder is a software engineer from Brookline, MA. His wife and three boys all love playing Nat's World. His web site is http://www.nedbatchelder.com.

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