[Python-Dev] PEP 246, redux

[Guido van Rossum]

> > [Alex]
> >> I'm saying that if, by mistake, the programmer has NOT
> >> registered the A->C adapter (which would be easily coded and work
> >> perfectly), then thanks to transitivity, instead of a clear and simple
> >> error message leading to immediate diagnosis of the error, they'll get
> >> a subtle unnecessary degradation of information and resulting
> >> reduction
> >> in information quality.

[Guido]
> > I understand, but I would think that there are just as many examples
> > of cases where having to register a trivial A->C adapter is much more
> > of a pain than it's worth; especially if there are a number of A->B
> > pairs and a number of B->C pairs, the number of additional A->C pairs
> > needed could be bewildering.

[Alex]
> Hm?

I meant if there were multiple A's. For every Ai that has an Ai->B you
would also have to register a trivial Ai->C. And if there were
multiple C's (B->C1, B->C2, ...) then the number of extra adaptors to
register would be the number of A's *times* the number of C's, in
addition to the sum of those numbers for the "atomic" adaptors (Ai->B,
B->Cj).

> > But I would like to see some input from people with C++ experience.
> 
> Here I am, at your service.
[...]
> It's in the running for the coveted "Alex's worst nightmare" prize,

Aha. This explains why you feel so strongly about it.

But now, since I am still in favor of automatic "combined" adaptation
*as a last resort*, I ask you to consider that Python is not C++, and
that perhaps we can make the experience in Python better than it was
in C++. Perhaps allowing more control over when automatic adaptation
is acceptable?

For example, inteface B (or perhaps this should be a property of the
adapter for B->C?) might be marked so as to allow or disallow its
consideration when looking for multi-step adaptations. We could even
make the default "don't consider", so only people who have to deal
with the multiple A's and/or multiple C's all adaptable via the same B
could save themselves some typing by turning it on.

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