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

Alex Martelli aleax at aleax.it
Wed Jan 12 00:33:22 CET 2005

On 2005 Jan 11, at 22:08, Phillip J. Eby wrote: ...

Yes, you're ALLOWED to stuff with NULL any field that isn't explicitly specified as NOT NULL.

But you should ONLY do so when the information is REALLY missing, NOT when you've lost it along the way because you've implemented adapter-chain transitivity: dropping information which you COULD have preserved with a bit more care (==without transitivity) is a violation of PRAGMATICS, of the BEST-EFFORT implication, just as it would be to drop packets once in a while in a TCP/IP stack due to some silly programming bug which was passed silently. This is again a misleading analogy. You are comparing end-to-end with point-to-point. I am saying that if you have a point-to-point connection that drops all packets of a particular kind, you should not put it into your network, unless you know that an alternate route exists that can ensure those packets get through. Otherwise, you are breaking the network.

But adaptation is not transmission! It's PERFECTLY acceptable for an adapter to facade: to show LESS information in the adapted object than was in the original. It's PERFECTLY acceptable for an adapter to say "this piece information is not known" when it's adapting an object for which that information, indeed, is not known. It's only CONJOINING the two perfectly acceptable adapters, as transitivity by adapter chain would do automatically, that you end up with a situation that is pragmatically undesirable: asserting that some piece of information is not known, when the information IS indeed available -- just not by the route automatically taken by the transitivity-system.

What happened here is not that either of the adapters registered is wrong: each does its job in the best way it can. The programming error, which transitivity hides (degrading the quality of information resulting from the system -- a subtle kind of degradation that will be VERY hard to unearth), is simply that the programmer forgot to register the direct adapter. Without transitivity, the programmer's mistake emerges easily and immediately; transitivity hides the mistake.

By imposing transitivity, you're essentially asserting that, if a programmer forgets to code and register an A -> C direct adapter, this is never a problem, as long as A -> B and B -> C adapters are registered, because A -> B -> C will give results just as good as the direct A -> C would have, so there's absolutely no reason to trouble the programmer about the trivial detail that transitivity is being used.

At the same time, if I understand correctly, you're ALSO saying that if two other adapters exist, A -> Z and Z -> C, THEN it's an error, because you don't know when adapting A -> C whether to go via B or via Z. Well, if you consistently believe what I state in the previous paragraph, then this is just weird: since you're implicitly asserting that any old A->?->C transitive adaptation is just as good as a direct A->C, why should you worry about there being more than one such 2-step adaptation available? Roll the dice to pick one and just proceed.

Please note that in the last paragraph I'm mostly trying to "reason by absurd": I do NOT believe one can sensibly assert in the general case that A->?->C is just as good as A->C, without imposing FAR stronger constraints on adaptation that we possibly can (QI gets away with it because, designed from scratch, it can and does impose such constraints, essentially that all interfaces "belong" to ONE single object -- no independent 3rd party adaptation, which may be a bigger loss than the constraints gain, actually).

I'm willing to compromise to the extent of letting any given adaptation somehow STATE, EXPLICITLY, "this adaptation is lossless and perfect, and can be used as a part of transitive chains of adaptation without any cost whatsoever". If we do that, though, the adaptation system should trust this assertion, so if there are two possibilities of equal minimal length, such as A->B->C or A->Z->C, with all the steps being declared lossless and perfect, then it SHOULD just pick one by whatever criterion, since both will be equally perfect anyway -- so maybe my reasoning by absurd wasn't totally absurd after all;-).

Would this compromise be acceptable to you?

Alex

More information about the Python-Dev mailing list