[Python-Dev] Requesting pronouncement on PEP 0424

[PJ Eby]

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.)