[Python-Dev] PEP 246, redux (original) (raw)

Guido van Rossum gvanrossum at gmail.com
Wed Jan 12 19:16:14 CET 2005

> [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/)

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