[Python-Dev] Requesting pronouncement on PEP 0424

PJ Eby pje at telecommunity.com
Tue Jul 31 05:25:28 CEST 2012

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.)

More information about the Python-Dev mailing list