Mixing HTML5 Canvas and Python
I do exactly what you have mentioned using Django on the server side and HTML5 canvas/javascript on the client side. I'm pretty happy with the results but would like to point out that what you do with a Canvas on the client side doesn't have anything to do with what you use on the server side for Python.
I have used both of these (Python and HTML5) seperately, however I'm keen to use the full power of Python over the web using HTML5 to draw things and handle the client side of things. I guess I'm looking for avenues to go down in terms of implementation. Here are some things I'd like to do if possible:

Have very interactive data, which will need to processed server-side by Python but displayed and locally manipulated by HTML5 Canvas.
Clickable components on the HTML5 Canvas which will communicate with the server side.

Is there an implementation that people can recommend? I.e. would Google App Engine be any good. Django? Pyjamas?
Thanks - apologies if this seems a little vague. I'm asking before trying one path to see if there is a heads-up to save time and effort.