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

Calvin Spealman ironfroggy at gmail.com
Sat Apr 28 15:13:29 CEST 2007

On 4/26/07, Guido van Rossum <guido at python.org> wrote:
> 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.
> I now have a proposal to allow overloading isinstance() and
> issubclass(), by defining special (class) methods on the second
> argument. See http://python.org/sf/1708353. Does this need a PEP? The
> unit test shows that it can be used to support the use case described
> above:
> class ABC(type):
>     def __instancecheck__(cls, inst):
>         """Implement isinstance(inst, cls)."""
>         return any(cls.__subclasscheck__(c)
>                    for c in {type(inst), inst.__class__})
>     def __subclasscheck__(cls, sub):
>         """Implement issubclass(sub, cls)."""
>         candidates = cls.__dict__.get("__subclass__", set())
>         return any(c in candidates for c in sub.mro())
> class Integer(metaclass=ABC):
>     __subclass__ = {int}
> --
> --Guido van Rossum (home page: http://www.python.org/~guido/)
> _______________________________________________
> Python-3000 mailing list
> Python-3000 at python.org
> http://mail.python.org/mailman/listinfo/python-3000
> Unsubscribe: http://mail.python.org/mailman/options/python-3000/ironfroggy%40gmail.com

I'm just going to jump into this and voice a concern that allowing
overriding of isinstance and issubclass seems like a Bad Idea. They
should be trustworthy and predictable, and allowing classes to change
how objects are considered instances of them seems like the wrong
thing to do. I'm not saying that I don't think it should be done, but
that it feels quite wrong. When I do, rarely, use isinstance or such,
and I need to check isinstance(f, Foo), I know exactly what I am
saying. But if Foo can change the meaning of that, can I trust it

Not everything should be dynamic. I am not sure which side of the
fence this falls on, but right now, in my mind, it teeters right on
the fence itself.

Read my blog! I depend on your acceptance of my opinion! I am interesting!

More information about the Python-3000 mailing list