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.
On Tue, Mar 3, 2020 at 04:03 Steve Jorgensen <ste...@stevej.name> wrote: > I have encountered cases in which I would like to validate that an > argument can be properly compared with other instances of its type. This is > true of numbers, strings, dates, … but not for `NoneClass`, `type`, …. > > One way that I have tried to handle this is to check whether the object > can be compared to itself using `>`, `<`, `>=`, and `<=` and that it is > neither `>` or `<` itself and is both `>=` and `<=` itself. The most > glaring example of why this is insufficient is the `set` type. A `set` > object meets all of those criteria, but given any 2 instances, it is not > true that if set `a > b` is `False` then `a <= b` is `True`. The operators > are not acting as comparisons of relative magnitude in this case but as > tests for superset/subset relations — which is fine and good but doesn't > help with this situation. > > What I think would be helpful is to have a `Magnitude` abstract base class > that is a subclass of `ProtoMagnitude` (or whatever better names anyone can > imagine). > > The subclass hook for `Magnitude` would return `True` for any class with > instance methods for all of the rich comparison methods, but it would skip > that check and return `False` for any real or virtual subclass of > `ProtoMagnitude` (overridable by registering as a `Magnitude` subclass). > The, `set` type would then be registered as a virtual base class of > `ProtoMagnitude` but not `Magnitude` so that `issubclass(set, Magnitude)` > would return `False`. > > For performance optimization, the module that defines these ABCs would > register the obviously appropriate built-in and standard-lib types with > `Magnitude`: `Number`, `str`, `list`, `tuple`, `date`, … > > Why not have this be a separate distribution package? This concept is only > reliable if all of the code that makes use of it shares a common > implementation. It does no good to register a class as `ProtoMagnitude`, > for instance, if an instance of that will passed to code in another library > that is unaware of the `ProtoMagnitude` and `Magnitude` ABCs in the > package, or maybe has its own independent system for attempting to > accomplish the same goal. > _______________________________________________ > 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/7WC4SF2GYVLP56K6Q74OKFPJGHGWAPIP/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- --Guido (mobile)
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