Recursive class | can you modify self directly?

Ian Kelly ian.g.kelly at
Wed Jul 10 00:18:00 CEST 2013

On Tue, Jul 9, 2013 at 4:01 PM, Russel Walker <russ.pobox at> wrote:
> 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:
> 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 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')

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