[Python-Dev] PEP 246, redux

[Clark C. Evans]

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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