[Python-Dev] Requesting pronouncement on PEP 0424

Guido van Rossum guido at python.org
Tue Jul 31 06:17:25 CEST 2012

On Tue, Jul 31, 2012 at 5:25 AM, PJ Eby <pje at telecommunity.com> wrote:
> On Mon, Jul 30, 2012 at 12:51 PM, Guido van Rossum <guido at python.org> wrote:
>> - Most importantly: calling len(obj) and catching TypeError can only
>> be a substitute for the real implementation, which IMO ought to check
>> for the presence of a tp_len slot. Alas, checking hasattr(obj,
>> '__len__') doesn't quite cut it either, since this returns true for a
>> class object that defines a __len__ method for its instances (the
>> class itself doesn't have a length).
> This isn't the only place this pattern comes up; maybe a hasmethod()
> function somewhere (builtin, operator, inspect?) for this would be a
> good idea.  (i.e., something that returns true only if the method is
> for the instance.)
> (But perhaps that's a python-ideas topic, since it raises the question
> of whether it should really be something more like instancehasattr(),
> or whether it should be limited to special slots or something else.)

Yes, please redirect / repost; I read p-ideas too. It's an interesting
topic, if very specialized.

--Guido van Rossum (python.org/~guido)

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