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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