On Wed, Aug 5, 2020 at 7:06 PM Steven D'Aprano <st...@pearwood.info> wrote:
> *blinks* > > When did this happen? > > I'm on Python-Ideas, Python-Dev, and I get announcements of new issues > on the bug tracker, and I don't recall ever seeing this feature > discussed. > Oddly I barely recall it either, even though according to PEP 487 it was posted five times to python-dev, and I approved it. It's a long time ago though (two retirements, for me :-). > [looks up the docs] > > Okay, apparently it was added in 3.6. But the documentation says: > > > """ > When using the default metaclass type, or any metaclass that ultimately > calls type.__new__, the following additional customisation steps are > invoked after creating the class object: > > first, type.__new__ collects all of the descriptors in the class > namespace that define a __set_name__() method; > """ > > https://docs.python.org/3/reference/datamodel.html#class-object-creation > > but that's not what is happening here, since my_property is not a > descriptor, it's just an arbitrary instance. > > (To be a descriptor, it needs to have `__get__` and/or `__set__` > methods.) > > Have I missed something or does this need a documentation fix? > That's a good observation. I think the PEP was only thinking of descriptors, and the implementation possibly took a shortcut by calling `__set_name__` on every attribute. Or perhaps the PEP was ambiguous, since under proposal, item (2) states that the "hook is called on all the attributes (descriptors) defined in the class". Now there's a general rule that says "if you use a dunder in a way that's undocumented, the behavior is undefined" (and this includes making up your own dunders), which means that technically the implementation could do whatever it wants -- but since this is Python we probably want to accept that it's called for every attribute, whether it smells like a descriptor or not, and it would be nice to fix the docs. Do you have the powers to submit PRs these days? -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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