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

Clark C. Evans cce at clarkevans.com
Thu Jan 13 18:09:15 CET 2005

On Fri, Jan 14, 2005 at 12:30:17AM +1000, Nick Coghlan wrote: | Anyway, I'd like to know if the consensus I think you've reached is the | one the pair of you think you've reached :)

This stated position is not the current PEP status, and it's not a concensus position I share.

| That is, with A being our starting class, C being a target class, and F | being a target interface, the legal adaptation chains are: | # Class to class | A->C | # Class to interface, possibly via other interfaces | A(->F)*->F

PEP246 should not talk about legal or illegal adaption chains, all adaption chains should be explicit, and if they are explicit, the programmer who specified them has made them legal.

| With a lookup sequence of: | 1. Check the global registry for direct adaptations | 2. Ask the object via conform | 3a. Check using isinstance() unless 2 raised LiskovViolation | 3b. Nothing, since object.conform does an isinstance() check | 4. Ask the interface via adapt

These are OK up to 4.

| 5. Look for transitive chains of interfaces in the global registry.

No! No! No! Perhaps...

5. Raise a AdaptionFailed error, which includes the protocol
   which is being asked for.  This error message _could_ also
   include a list of possible adaptation chains from the global
   registry, but this is just a suggestion.

| 3a & 3b are the current differing answers to the question of who should | be checking for inheritance - the adaptation machinery or the conform | method.

Correct. I think either method is OK, and perfer Phillip's approach.

Best,

Clark

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