Recursive class | can you modify self directly when using python

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?

Answer 1

"self" is just a local binding of the object to the name self. You can rebind the name like this as with any other local variable, but that's all it does. It doesn't modify the object in any way, and no other bindings of the same object are affected.

If you actually want to modify the current object, you would need to do something like:

 def expand(self):
 import copy
 self.expr = Expr(self.expr, self.op, self.val)
 self.op = choice('+-*/')
 self.val = choice('12345')

Comments

Minus the "import copy".