Steve Jorgensen wrote: <snip> > The problem I came up with trying to spike out my proposal last night is that > there > doesn't seem to be anyway to implement it without creating infinite recursion > in the > issublcass call. If I make Orderable a real or virtual subclass > of ProtoOrderable and Orderable's __subclasshook__ > or metaclass __subclasscheck__ (I tried both ways) tries to check whether > C is a subclass of ProtoOrderable, then an infinite recursion > occurs. > It wasn't immediately obvious to me why that is the case, but when I thought > about it > deeply, I can see why that must happen. > An alternative that I thought about previously but seems very smelly to me > for several > reasons is to have both Orderable and NonOrderable ABCs. In that > case, what should be done to prevent a class from being both orderable and > non-orderable > or figure out which should take precedence in that case? > As a meta-solution (wild-assed idea) what if metaclass registration could > accept > keyword arguments, similar to passing keyword arguments to a class > definition? That way, a > single ABC (ProtoOrderable or whatever better name) could be a real or > virtual subclass that is explicitly orderable or non-orderable depending on > orderable=<True/False>.
I have been unable to implement the class hierarchy that I proposed, and I think I've determined that it's just not a practical fit with how the virtual bas class mechanism works, so… Maybe just a single `TotalOrdered` or `TotalOrderable` ABC with a `register_explicit_only` method. The `__subclasshook__` method would skip the rich comparison methods check and return `NotImplemented` for any class registered using `register_explicit_only` (or any of its true subclasses). The only weird edge case in the above is that is someone registers another ABC using `TotalOrdered.register_explicit_only` and uses that as a virtual base class of something else, the `register_explicit_only` registration will not apply to the virtual subclass. I'm thinking that's completely acceptable as a known limitation if documented? _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/AF4O3PFQ7VNHCUVBWB3NENYNGPU74SVX/ Code of Conduct: http://python.org/psf/codeofconduct/
