Tcl style traces

[Derek Fountain]

> What do you need this for?  If you can be more specific, we might be able
> to help you a little better.

I have a Tcl/Tk script which uses a canvas to graphically represent the data
stored in the program. Whenever the data store is updated, a Tcl trace
automatically triggers to ensure the graphical display is kept consistent
with the data. I was after a simple way to convert the script to Python
Tkinter. Without the eqivalent of Tcl traces I'm probably going to have to
think about it... ;o)