Guido van Rossum wrote: > On Tue, Mar 3, 2020 at 10:43 AM Steve Jorgensen ste...@stevej.name wrote: > > Guido van Rossum wrote: > > I think it’s usually called Orderable. It’s a > > useful concept in static > > type > > checking too (e.g. mypy), where we’d use it as an upper bound for type > > variables, if we had it. I guess to exclude sets you’d have to introduce > > TotalOrderable. > > Right. That's a much better term. Orderable and > > ProtoOrderable. > > Or even PartialOrderable and Orderable. This would follow Rust's > PartialOrd > and Ord (https://doc.rust-lang.org/std/cmp/trait.PartialOrd.html > and > https://doc.rust-lang.org/std/cmp/trait.Ord.html). > But beware, IIRC there are pathological cases involving floats, (long) ints > and rounding where transitivity may be violated in Python (though I believe > only Tim Peters can produce an example :-). I'm honestly not sure that > that's enough to sink the idea. (If it were, NaN would be a bigger problem.)
Yeah. Violations of transitivity are already breaking their contracts, so having a new way of expressing the contract has no affect on that. 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>`. _______________________________________________ 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/BFFPWPZGNPJCT3KFFU6DJHI5RBG2NBYC/ Code of Conduct: http://python.org/psf/codeofconduct/
