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

Guido van Rossum guido at python.org
Sat Apr 28 20:57:37 CEST 2007

On 4/28/07, Calvin Spealman <ironfroggy at gmail.com> wrote:

On 4/28/07, Guido van Rossum <guido at python.org> wrote: > On 4/28/07, Jean-Paul Calderone <exarkun at divmod.com> wrote: > > Aside from the way in which `x' can already lie: > > > > >>> class X(object): > > ... class = property(lambda self: int) > > ... > > >>> isinstance(X(), int) > > True > > >>> > > > > Is this behavior changed/going to be changed in Py3k? > > I'm not particularly enamored with it, but I believe it once served a > purpose for Zope. Does anyone know if it is still needed? > > -- > --Guido van Rossum (home page: http://www.python.org/~guido/)

Consider this example: >>> class X(object): pass >>> class Y(object): a = 10 >>> x = X() >>> x.class = Y >>> x.a 10 >>> isinstance(x, Y) True Now, I'm not saying we must keep this behavior, but it does have its merits. In particular, the cases of type-changing objects, which I'm not convinced always have a place, but thats not the point. The point I want to make is that x is not lying about its type. It IS a Y, because class determines its type. The breakage may not be that x could lie and say its an int, but that it actually can't its a deception of the class, not the type, because you can't assign x.class = int, for example. Either the behavior should work for any type, thus allowing heap types to be assigned to class attributes, changing the type of objects (but, obviously immutables would disallow class assignment). Or, an objects type should not be determined simply by the value of attributes, class, at all. Does that make sense?

Alas not much. Your code example is about actually changing the type of an object, which is a completely different issue, and is constrained by the object lay-out (try changing the class where both classes use different slots).

Overriding isinstance is about what classes claim that the object is an instance of them, not for the purpose of changing the object's behavior or representation, but purely for the sake of API introspection.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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