How does Subclassing Work?

[s713221 at student.gu.edu.au]

Recently I tried to write a piece of code that subclassed Canvas.py from
the Tkinter distribution, where I overwrote CanvasItem. Now I was hoping
that all calls to Arc, Line etc. (which weren't overwritten) would call
my modified CanvasItem, but it seems they were calling the unmodified
CanvasItem - which isn't what I wanted.

What I wanted was:

My module	  ______> calls		CanvasItem
		  |
Canvas.py	Arc			CanvasItem

What I got was:

My module				CanvasItem

Canvas.py	Arc _______> calls	CanvasItem

Other than copy all of Canvas.py into my module and redefine everything,
is there a way I can get this to work as expected without rewriting
Canvas.py? What I was trying to do was alter Canvas so that I could
create classes that would make complex items.

Joal Heagney/AncientHart