[Python-3000] weakrefs of bound methods

[Guido van Rossum]

On Fri, Feb 15, 2008 at 10:26 AM, Phillip J. Eby <pje at telecommunity.com> wrote:
> At 09:35 AM 2/15/2008 -0800, Guido van Rossum wrote:
>  >On Fri, Feb 15, 2008 at 2:12 AM, Nick Craig-Wood <nick at craig-wood.com> wrote:
>  > > I've just been bitten yet again by the fact you can't have a weakref
>  > >  to a bound method!  I find myself wanting to do this whenever I have a
>  > >  registry of callback functions.
>  > >
>  > >  Could we in py3k either
>  > >
>  > >  1) make weakrefs to bound methods work?  A weakref to a bound method
>  > >  should mean hold the weakref on the instance and bind the method at
>  > >  the last moment.
>  >
>  >I think you're somehow confused about how weakrefs are supposed to
>  >work. They work fine!
>
>  The use case here is actually to keep the callback around only so
>  long as the object exists.

Ah, I see. Nick actually wrote this but because I'm so familiar with
the concept of a bound method object I didn't see what his actual
issue was. Sorry Nick!

>  There are various ways to do this, of course, that don't require
>  changing the language or stdlib.  One of the simplest (for 2.4 and
>  up) is to create a callable weakref subclass that emulates a bound
>  method.  Another is to create a bound method whose im_self is a weakref proxy.
>
>  I've never actually encountered a usecase for keeping a standard
>  weakref to a standard bound method, though.  It's pretty useless,
>  because bound methods are nearly always immediately disposed of, and
>  there's no logical place (or reason) to store them.  So, Nick's query
>  is (in effect), can we either:
>
>  1. make ref(method) create a "weak method" instead, or
>
>  2. make ref(method) an error instead of useless behavior that gives
>  you the illusion of working code

#2 is easily implemented (just remove the machinery to support weak
refs from the bound method object) but the question is, will it break
anybody's code -- and if so, do we care enough to break it? (In Py3k,
that is.)

#1 isn't so easy in terms of how you'd implement it -- I don't think
the weakref.ref() call currently has a way to customize what kind of
thing you get back. I'd be open for a patch containing working code
that can be generalized to other cases.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)