Andrew Barnert wrote:
> On Mar 5, 2020, at 11:05, Steve Jorgensen ste...@stevej.name wrote:
> > Steve Jorgensen wrote:
> > 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.
> > Is this something we should be looking to add to the ABC mechanism in
> general?
> Would a way to “unregister” classes that would be implicitly accepted be
> simpler than a
> way to “register_explicit_only” classes so they skip the implicit test?
> > 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?
> > Code spike of that idea:
> > from abc import ABCMeta
> > from weakref import WeakSet
> >
> >
> > class TotallyOrderable(metaclass=ABCMeta):
> > _explicit_only_registry = WeakSet()
> >
> > @classmethod
> > def register_explicit_only(cls, C):
> > if cls is not TotallyOrderable:
> > raise NotImplementedError(
> > f"{cls.__name__} does not implement
> > 'register_explicit_only'")
> >
> > cls._explicit_only_registry.add(C)
> >
> > @classmethod
> > def __subclasshook__(cls, C):
> > if cls is not TotallyOrderable:
> > return NotImplemented
> >
> > for B in C.__mro__:
> > if B in cls._explicit_only_registry:
> > return NotImplemented
> >
> > return cls._check_overrides_rich_comparison_methods(C)
> >
> > @classmethod
> > def _check_overrides_rich_comparison_methods(cls, C):
> > mro = C.__mro__
> > for method in ('__lt__', '__le__', '__gt__', '__ge__'):
> > for B in mro:
> > if B is not object and method in B.__dict__:
> > if B.__dict__[method] is None:
> > return NotImplemented
> > break
> > else:
> > return NotImplemented
> > return True
> >
> >
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> >
Maybe so because I found a limitation with my code spike. Calling
`register_explicit_only` doesn't bust the cache, so if `issubclass(<myclass>,
TotallyOrderable)` is called prior to calling
`TotallyOrderable.register_explicit_only(<myclass>)`, then it doesn't have any
effect. Perhaps, that's also not a big deal as a documented limitation though?
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