[Python-3000] ABC PEP isinstance issue Was: PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities)

[Guido van Rossum]

This is a very good point. Perhaps we can come up with a way to make
isinstance and issubclass into something like GFs (without requiring
the whole GF machinery).

I'll think about it some more.

--Guido

On 4/25/07, Jim Jewett <jimjjewett at gmail.com> wrote:
> The current ABC proposal is to use isinstance as the test; Jeffrey
> Yaskin's numbers PEP highlighted the weakness there with a concrete
> example.
>
> If you need to an abstraction less powerful than an existing ABC,
> you're out of luck; you can't just assert that the existing class is
> already sufficient, nor can you expect everyone else to use multiple
> annotations.
>
> Short of allowing more __bases__ surgery, we need a function parallel
> to isinstance, which at least makes 3rd-party registration possible.
> I suspect Phillip will say that we really need to make the ABCs
> generic functions... but I disagree; I'm not sure that 3rd-party
> adapters should even be allowed by default, but it *should* be simple
> to create an ABC that does take them.
>
> Perhaps
>
> def isexample(obj, ABC):
>     for cls in obj.__class__.__mro__:
>         result = ABC.meets(obj, cls)
>         if result:
>             return result
>     return False
>
> class Abstract...
>
>     # override this with a dictionary to allow 3rd-party registration
>     _good_enough = ()
>
>     @classmethod
>     def meets(cls, obj=None, objclass=None):
>         if objclass is cls:   # covers isinstance
>             return obj
>         if objclass in cls._good_enough:   # Nothing is, by default
>             return cls._good_enough.[objclass](obj)
>
>     @classmethod
>     def assert_sufficient(cls, objclass, adapter):
>         cls[objclass]=adapter
>
>
> On 4/25/07, Jeffrey Yasskin <jyasskin at gmail.com> wrote:
>
> > If someone needs to split them later, they can use code like::
> >     import numbers
> >     class IntegralDomain(Ring): ...
> >     numbers.Integral.__bases__ = (IntegralDomain,) + numbers.Integral.__bases__
>
> This only works with old-style classes, which are going away.
>
> >>> class Abstract1(object): pass
> >>> class Abstract2(object): pass
> >>> Abstract1.__bases__ = (Abstract2,) + Abstract1.__bases__
>
> Traceback (most recent call last):
>   File "<pyshell#33>", line 1, in <module>
>     Abstract1.__bases__ = (Abstract2,) + Abstract1.__bases__
> TypeError: __bases__ assignment: 'Abstract2' deallocator differs from 'object'
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)