[Python-Dev] ABCs and MRO

[Jeffrey Yasskin]

On Mon, Mar 2, 2009 at 2:41 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> ...
> More generally, there is NO WAY to determine the list of classes for
> which issubclass(C, x) is true.
>
> This could be considered a limitation of, or a bug in, ABCs, I don't
> have a particular opinion on that, but it does mean that no code which
> relies on being able to traverse the class inheritance graph will see
> ABCs. One particular case of this is (any implementation I can think
> of, of) generic functions.
>
> In my view, this implies one of the following:
>
> 1) It should be a documented limitation of such code that it doesn't
> work with ABCs (and conversely, this limitation of ABCs should be
> documented in the ABC documentation)
> 2) Generic functions, and any other code requiring this type of
> introspection, is essentially useless unless it can support ABCs, and
> should not be used in the light of this limitation.
> 3) This is a bug in ABCs and should be fixed.
> 4) Something else I didn't think of :-)

I tend to think it's a bug in ABCs. You seem to have thought of
several possible ways to fix it, and I don't have strong preferences
between them.

One way to deal with __subclasscheck__ definitions other than
ABCMeta.__subclasscheck__ would be to make the "someone registered a
subclass" interface public in addition to the "tell me when someone
registers a subclass" interface. Then other __subclasscheck__
definitions could call the "someone registered a subclass" method when
the class is defined if they want to be usable with generic functions.

You can get something similar to the mro by checking whether ABCs are
subclasses of each other.

Jeffrey