[Python-Dev] PEP 246, redux

[Alex Martelli]

On 2005 Jan 12, at 16:45, Guido van Rossum wrote:

> My personal POV here: even when you break Liskov in subtle ways, there
> are lots of situations where assuming substitutability has no ill
> effects, so I'm happy to pretend that a subclass is always a subtype
> of all of its base classes, (and their base classes, etc.). If it
> isn't, you can always provide an explicit adapter to rectify things.

Ah, this is the crucial point: an explicit adapter must take precedence 
over substitutability that is assumed by subclassing.  From my POV this 
does just as well as any other kind of explicit control about whether 
subclassing implies substitutability.

In retrospect, that's the same strategy as in copy.py: *FIRST*, check 
the registry -- if something is found in the registry, THAT takes 
precedence.  *THEN*, only for cases where the registry doesn't give an 
answer, proceed with other steps and checks and sub-strategies.

So, I think PEP 246 should specify that the step now called (e) 
[checking the registry] comes FIRST; then, an isinstance step 
[currently split between (a) and (d)], then __conform__ and __adapt__ 
steps [currently called (b) and (c)].  Checking the registry is after 
all very fast: make the 2-tuple (type(obj), protocol), use it to index 
into the registry -- period.  So, it's probably not worth complicating 
the semantics at all just to "fast path" the common case.

I intend to restructure pep246 at next rewrite to reflect this "obvious 
once thought of" idea, and thanks, Guido, for providing it.


Alex