[Python-Dev] PEP 246, redux

[Nick Coghlan]

Alex Martelli wrote:
> "I really wish the language had 
> private inheritance because I'm using Abstract as a base just for code 
> reuse"

Funny you should say that. . . what about a __nonconformant__ entry that accepts 
a list of base classes that is used to indicate inheritance without a proper 
is-a relationship?

So if the isinstance check succeeds and there is no '__nonconformant__' entry, 
then adapt() just returns the object.

If, on the other hand, __nonconformant__ is supplied, then adapt() can check the 
list of base classes that remains after removing the entries in 
__nonconformant__ to see if the object would *still* be an instance a subtype, 
even after removing the noncomformant bases, and if it is, return it.

Otherwise, continue on to the rest of the adaptation process.

This should give a better idea what I mean:

# The 'fast path'
if isinstance(obj, protocol):
   if not hasattr(obj, "__nonconformant__"):
     return obj
   conformant_bases = set(obj.__bases__) - set(obj.__nonconformant__)
   for base in conformant_bases:
     if issubtype(base, protocol):
       return obj
# Continue on with the other adaptation possibilities (including __conform__)


Then you can get 'discreet' inheritance (you have the methods, but you don't 
brag about the fact) by writing:

class Dubious(Abstract):
   __noncomformant__ = [Abstract]
   # etc

rather than:

class Dubious(Abstract):
   def __comform__(self, protocol):
     if issubtype(protocol, Abstract):
       raise LiskovViolation
   # etc

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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