OO design question / Transform object in place?

[Dave Benjamin]

andy2O at hotmail.com wrote:
> Now suppose I set "expression2 = Sum([a,-a])" and Sum.simplify()
> recognises that the two terms cancel and the Sum has value 0.
> 
> Can I make "expression2.simplify()" transform expression2 from an
> instance of Sum to an instance of Number(0) **in place**? Is that
> possibe, or do I really have to write

I think it's much better for simplify() to return a new object always, 
and leave the original object unmodified. You can still write:

expression2 = expression2.simplify()

if you don't care about the old value.

> expression2 = SimplifyFunction(expression2)

This is another valid solution, but it's not an OO solution (you will 
need to use "isinstance" to write "SimplifyFunction").

> and use the function return to set expression2 to be a Number(0)
> object, which is annoying for a variety of reasons! Have I made a
> mistake in the design?

It's usually considered poor OO style to have an object change its class 
at runtime, although I'm sure you could do it in Python ("__bases__" 
would be a place to start) if you really wanted. For what you're trying 
to do, I don't think it's necessary, though.

Dave