I'm thinking OOP, more than any other paradigm, encourages a rich set of metaphors, which may be touted as an advantage if you're ever up against the well in an OOP-hostile room (been there done that). "Anti-imperativists" can be pretty militant. Inheritance is a most obvious metaphor, from whence come any number of ancestor, parent, child, multi-parent paradigms. All that assumed knowledge from "real life" infuses the shop talk, giving a boost in understanding provided the metaphors aren't too misleading. The Turtle has become ubiquitous as an obvious "self" in some environment that might in turn be subclassed if we like. Some joke about "turtles all the way down". It's the ultimate avatar or sim, usually seen as "3rd person" but could be "1st person" as well (change of viewpoint). However, I went with a Tractor instead of Turtle precisely because the turtle metaphors have been milked pretty thoroughly and needn't be the last word. With a Tractor, we get new etymologies, such as traction, tractare, to drag, to plow, to leave a rut or mark, to record, to do work (tractatus). One of the easiest ways to show off metaphors is not to subclass a the Tractor type however, but to simply rename it. Generic:

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from farmworld import Tractor as Worker, Farm as Factory

More specific:

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from farmworld import Tractor as government_worker, Farm as Area_51

Students immediately appreciate the analogies between a horse-like fuel burning engine that does the work of hundreds of humans, being akin to some generic "doer" or "agent", with the Farm being akin to said worker's "environment" or "workplace". One is starting to think in a more OO fashion when one can mentally juxtapose what is common among objects. The Tractor class simply "mows the lawn" in an XY grid. Actually, a tractor leaves no record in the field itself (self.field is a list of lists), unless you invoke the plow method (why not plough = plow?) to make a mark (replace an ASCII character with a different one -- peek and poke come to mind, from old timer BASIC coders). Ed asked me if I'd considered Unicode for the Farm cells. Indeed, a big part of the Lessons is to mentally add a 3rd dimension to each cell. Given this is a Farm, I use the word silo, which has the interesting property of pointing either up, above the plane (a grain silo), or below (a missile silo). If this XY grid were a bitmap (BMP), or tif file, with each cell a pixel, then the silo could be about color depth. As it is, even though we're getting by with Latin-1, we can suggest a 32-bit deep silo, with various UTF encodings compressing that 32-bit pattern. The mapping to ASCII is suggestive, and the fact that we're getting by with single bytes merely highlights the primitivist aesthetic across all modules. The Tractor then needs to be subclassed. It's still an iterator, meaning it exports the iterator interface (the Java way of talking) with __iter__ and __next__. The Sensor tractor does something most Logo-based turtles don't do: it senses its environment (of course Logo lets you write these procedures right? You can query the color value at a screen position?). The Sensor tractor doesn't just look straight down, at the cell over which it sits -- (Y, X) -- it looks in the 8 directions perpendicular to the edges of a Stop sign: NW N NE E SE S SW W. If any of these happen to be off the field, there's a default value. Tractors know to StopIteration when they hit a field boundary or fence. With a Sensor Tractor, we can play Conway's Game of Life, by checking the neighborhood. Rather than subclass again, we just write a function taking Sensor tractors for granted. With a Sensor Tractor, we may also implement Wolfram's 256 rules, simply by reading the three cells to the north (NW N NE) and following some simple substitution rules. http://4dsolutions.net/ocn/python/OST/lifegame.py http://4dsolutions.net/ocn/python/OST/nks.py The CropCircle Tractor is different. It is inwardly driven by a generator for the Mandelbrot Set (others might be swapped in, as the "brains" for this Tractor). The CropCircle subclass stretches a complex plane of given size over whatever 2d array (list of lists). When its plow method is fired, it doesn't just make a mark, like the ancestor / parent. It computes a mark based on what the Mandelbrot Set would be here, after so many iterations of its generator. The resulting output, after the field has been mowed, is a Mandelbrot in rough outline, a primitive example of "ASCII art": http://4dsolutions.net/ocn/python/OST/ostcanvas.py http://www.flickr.com/photos/17157315@N00/5645244292/in/set-7215762564607179... Kirby Much more about Tractors etc.: http://groups.google.com/group/mathfuture/browse_thread/thread/d2390406c11f8...