[Python-Dev] Requesting pronouncement on PEP 0424 (original) (raw)

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 tplen 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.)

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