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

Alex Martelli aleax at aleax.it
Wed Jan 12 20:31:25 CET 2005

On 2005 Jan 12, at 19:16, Guido van Rossum wrote: ...

[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).

Ah, OK, I get it now, thanks.

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?

Yes, that would be necessary to achieve parity with C++, which does now have the 'explicit' keyword (to state that a conversion must not be used as a step in a chain automatically constructed) -- defaults to "acceptable in automatic chains" for historical and backwards compatibility reasons.

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.

Yes, this idea you propose seems to me to be a very reasonable compromise: one can get the convenience of automatic chains of adaptations but only when the adaptations involved are explicitly asserted to be OK for that. I think that the property (of being OK for automatic/implicit/chained/transitive use) should definitely be one of the adaptation rather than of an interface, btw.

Alex

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