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

[Guido van Rossum]

On 4/29/07, Jim Jewett <jimjjewett at gmail.com> wrote:
> Guido wrote:
> > Note though that only the second argument to either function can
> > overload the rules. IOW if you write isinstance(x, C), there is no way
> > that x could attempt to lie; but C could.
>
> As Barry pointed out, this means a class can't easily say "my parent
> may well implement ABC, but I don't."  That special case could be
> added to the default overriding behavior, but ... it starts to get
> fragile.

That use case is very questionable, and I see it more as an
anti-pattern; I suspect that it's a case of inheriting implementation
where you really ought to be using containment instead of subclassing.
It's why I'm uncomfortable with string.Template inheriting from a
string type, and and ditto for the OO os.path proposals that have the
path object inherit from a string type (I know not all of them do).

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