> On 24 Jul 2015, at 17:29, Nick Coghlan <ncogh...@gmail.com> wrote: > > On 25 July 2015 at 00:50, Brett Cannon <br...@python.org> wrote: >> Leave the decorator out like __new__, otherwise people are bound to forget >> it and have a hard time debugging why their code doesn't work. > > I'd actually advocate for keeping this as a metaclass method, rather > than making it available to any type instance. The key thing to > consider for me is "What additional power does making it a method on > the class itself grant to mixin types?”
To be honest, I hadn’t considered mixin types yet. > > With PEP 487, the __init_subclass__ proposal only grants mixins the > power to implicitly run additional code when new subclasses are > defined. They have no additional ability to influence the behaviour of > the specific class adding the mixin into the inheritance hierarchy. > > With PEP 447, as currently written, a mixin that wants to alter how > descriptors are looked up will be able to do so implicitly as long as > there are no other custom metaclasses in the picture. As soon as there > are *two* custom metaclasses involved, you'll get an error at > definition time and have to sort out how you want the metaclass > inheritance to work and have a chance to notice if there are two > competing __getdescriptor__ implementations. > > However, if __getdescriptor__ moves to being a class method on object > rather than an instance method on type, then you'll lose that > assistance from the metaclass checker - if you have two classes in > your MRO with mutually incompatible __getdescriptor__ implementations, > you're likely to be in for a world of pain as you try to figure out > the source of any related bugs. That’s a good point, and something that will move something that I’ve wanted to look into forward on my list: the difference between a classmethod and a method on the class defined through a metaclass. The semantics I’d like to have is that __getdescriptor__ is a local decision, defining __getdescriptor__ for a class should only affect that class and its subclass, and shouldn’t affect how superclasses are handled by __getattribute__. That is something that can be done by defining __getdescriptor__ on a metaclass, and AFAIK requires active cooperation when using a @classmethod. It should be possible to demonstrate the differences in a pure Python prototype. Ronald > > Cheers, > Nick. > > -- > Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > https://mail.python.org/mailman/options/python-dev/ronaldoussoren%40mac.com _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
