[Python-3000] iostack and Oh Oh (original) (raw)

Guido van Rossum guido at python.org
Wed Dec 6 23:11:18 CET 2006

On 12/6/06, Phillip J. Eby <pje at telecommunity.com> wrote:

I would also like to point out again, though, that the mechanism I've proposed (wrapping generic functions to create interfaces) actually supports (or can be made to support, using appropriate convenience functions) all the use cases that have been put forth for interfaces, adaptation, and generic functions. At least, the use cases I understand.

Would you mind going through the set example in detail? Or don't you understand that use case?

To support interface inspection in the "mapping vs. sequence", my proposed approach requires that you define a query operation like ismapping() or issequence(). This is indeed more work. My argument, however, is that this is a good thing, because interface inspection in the general case is evil: it leads to the unfixable contextual interpretations problem, whenever two developers write code that inspects the same interface for different purposes. So, I think it's perfectly acceptable to make people jump through an extra hoop in order to do that sort of thing, especially since it's rare that you're going to have a mapping vs. sequence type of problem.

Why do you think developers would abuse interfaces but they wouldn't abuse the meaning of ismapping() or issequence()? it seems that this doesn't help at all -- whether we call an inquiry function with a vague meaning or we have an interface with a vague meaning, misunderstandings are right around the corner, unless we provide as many different query operations as there are semantic variants. And we could solve that with "classic" interfaces just as well as with your proposal.

And the query operation doesn't help for adaptation, unless you go all the way and implement essentially PyProtocols' "when ", which I'd like to keep out of the core language and library. I also don't like the idea that the query operation can return a dynamic value -- it could decide whether to be a sequence or mapping (or both) based on the time of day, or the contents of an object, or anything else. That's more than I bargained for -- I'm only interested in invariants that the class cares to provide.

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

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