OO design question / Transform object in place?

[andy2O at hotmail.com]

Hello comp.lang.py,

Can you help me with ideas for the following (somewhat newbie) OO
design question in Python? Note, I'm using psuedo-code, not actual
Python for the examples!

Background:
-----------
I need to represent a small variety of mathematical constructs
symbolically using Python classes.

1) I have classes, representing numbers, symbolic variables etc.
2) I have classes for groups of these objects, such as a symbolic Sums.
3) To simplify the mathematical constructs, I've provided each of the
   classes with a simplify(self) method.

The following psuedo-code gives an example

a = Variable('a')
b = Variable('b')
two = Number(2) #Yes there are good reason not
                #to just use Python's int object! :-)

expression1 = Sum([a,b,a,a,two]) #This represents a+b+a+a+2

then calling "expression1.simplify()" would invoke Sum.simplify() and
reduce expression1 to 3*a+b+2

Question:
---------

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

expression2 = SimplifyFunction(expression2)

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?

Many thanks,
andy.