make an object read only

[Robin Becker]

A reportlab user found he was doing the wrong thing by calling canvas.save 
repeatedly, our documentation says you should not use Canvas objects after the 
save method has been used. The user had mixed results :(

It would be better to make the canvas object completely immutable all the way 
down when save has been called, but I think that's quite hard as these objects 
have quite a large and varied set of attributes, lists other objects 
dictionaries etc etc.

I can think of some strategies for making the object unusable eg changing it's 
class and getting rid of __dict__, but that disallows referencing valid 
properties eg pagesize, fontName, etc etc.

Is there a way to recursively turn everything immutable?
-- 
Robin Becker