[Python-3000] Adaptation vs. Generic Functions

[Guido van Rossum]

On 4/5/06, Walter Dörwald <walter at livinglogic.de> wrote:
> Guido van Rossum wrote:
>
> > On 4/5/06, Tim Hochberg <tim.hochberg at cox.net> wrote:
> >> I'm hoping that Walter can give some more realistic examples since I
> >> don't have real-world experience here. The basic idea is simply to let
> >> an adapter give up and let the protocol try the next adapter. This could
> >> happen in a generic function, for instance, if you wanted to try some
> >> fast algorithm for some specific subtypes, but the algorithm might be
> >> inappropriate depending on the values of the subtypes. You don't find
> >> that out till your in the adapter itself.
> >
> > Hm... The alternative, of course, instead of some magic feature, is to
> > explicitly call the fallback algorithm. That seems totally reasonable
> > for this example, and is similar to Python's single-inheritance-style
> > explicit super-call e.g. return BaseClass.methodName(self,
> > arguments...).
>
> For methods you know what your bases classes are,

Not necessarily in the case of multiple inheritance; that's why Python
2.2 added super().

>  but for adapters
> that's not necessarily so. Somewhat could have registered another
> adapter for the base class.

> > Does anybody know if Phillip Eby's generic functions support this?
>
> If I understood Ian's posting
> (http://mail.python.org/pipermail/python-3000/2006-April/000481.html)
> correctly, they do.
>
> > I
> > would think that the definition of "next candidate" can be problematic
> > there, in case you've broken a tie between two ambiguous possibilities
> > by registering an explicit tie-breaker. Remove the tie-breaker and
> > neither alternative dominates the other, so the next candidate is
> > undefined. This is different from Python's MRO calculations, which
> > always create a perfect linearization of the base classes.
>
> BTW, another useful feature might be the ability to copy the protocol.
> Then I could register a few other adapters for the copy and use the
> modified copy for my own purpose.

Let's not go wild with features; let's first collect use cases.

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