[Python-Dev] PEP 487 vs 422 (dynamic class decoration)

[Nick Coghlan]

On 4 Apr 2015 00:29, "Martin Teichmann" <lkb.teichmann at gmail.com> wrote:
>
> > When I first wrote PEP 422 I was of the view that "Python 2 allows
> > class definition postprocessing injection, we should allow it in
> > Python 3 as well". I've since changed my view to "Having to declare
> > post-processing of a class definition up front as a decorator, base
> > class or metaclass is a good thing for readability, as otherwise
> > there's nothing obvious when reading a class definition that tells you
> > whether or not postprocessing may happen, so you have to assume its
> > possible for *every* class definition".
>
> Nick, I couldn't agree more with you, yet I think PJ actually brought
> up a very interesting point. Post-processing is a very common thing
> these days, and has been re-written so many times that I think it is
> about time that something like it should be in the standard library.
>
> I'm less thinking about decorated methods, more about descriptors.
> They always have the problem that they don't know which attribute they
> belong to, so every author of a framework that defines descriptors
> writes a metaclass which goes through all the descriptors and tells
> them their attribute name.

Extending the descriptor protocol to include a per-descriptor hook that's
called at class definition time sounds like a potentially nice way to go to
me. While you *could* still use it to arbitrarily mutate the class object,
it's much clearer that's not the intended purpose, so I don't see it as a
major problem.

Cheers,
Nick.
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