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

Clark C. Evans cce at clarkevans.com
Wed Jan 12 00:38:58 CET 2005

On Tue, Jan 11, 2005 at 03:30:19PM -0500, Phillip J. Eby wrote: | Clark said he didn't want to assume substitutability; I was pointing out | that he could choose to not assume that, if he wished, by implementing an | appropriate conform at the base of his hierarchy.

Oh, that's sufficient. If someone making a base class wants to assert that derived classes should check compliance (rather than having it automagic), then they can do this. Good enough!

| I don't agree with Clark's use case, but my | proposal supports it as a possibility, and yours does not.

It was a straw-man; and I admit, not a particularly compelling one.

| To implement a Liskov violation with my proposal, you do exactly the same | as with your proposal, except that you can simply return None instead | of raising an exception, and the logic for adapt() is more | straightforward.

I think I prefer just returning None rather than raising a specific exception. The semantics are different: None implies that other adaptation mechanisms (like a registry) could be tried, while LiskovException implies that processing halts and no further adaptation techniques are to be used. In this case, None is the better choice for this particular case since it would enable third-parties to register a wrapper.

Overall, I think both you and Alex are now proposing essentially the same thing... no?

Best,

Clark

-- Clark C. Evans Prometheus Research, LLC. http://www.prometheusresearch.com/ o office: +1.203.777.2550 ~/ , mobile: +1.203.444.0557 // (( Prometheus Research: Transforming Data Into Knowledge \ , / - Research Exchange Database /\ - Survey & Assessment Technologies ` \ - Software Tools for Researchers ~ *

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