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Date: Tue, 09 Jul 2013 15:19:00 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: Recursive class | can you modify self directly?
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On 07/09/2013 03:01 PM, Russel Walker wrote:
>
> 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'))

`self` is just a name.  In `expand()` you are rebinding the name `self` away from the object and to a new Expr instance. 
  If you want to change `self` the original object you have to do something like:

     def expand(self):
         self.op = choice('+-*/')
         self.val = choice('12345')

--
~Ethan~