What is the scope of class variables in python? Does the self prefix modify this scope?

Question

Sorry for the vague title. Probably best to just show you the code that explains it better.

This is a simplified example of what I want to do:

# THIS DOESN'T WORK
from random import choice

class Expr(object):
 """
 Expr(expr, op, val) -> an expression object.
 """

 def __init__(self, expr, op='', val=''):
 self.expr = expr # can be another instance of Expression.
 self.op = op
 self.val = val

 def __str__(self):
 return ("%s %s %s" % (self.expr, self.op, self.val)).strip()

 def expand(self):
 self = Expr(self, choice('+-*/'), choice('12345'))

Then I tried messing around with Expr.__new__() and Expr.__init__() but that didn't work.

The only way I've got it to work is by doing the expanding part outside of the object, by a method of some other class. But I want the Expr class to be responsible for itself. How can I do this and why doesn't the above work?