[Python-3000] Adaptation vs. Generic Functions

[Walter Dörwald]

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, 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.

Servus,
    Walter