[Python-Dev] PEP 246, redux

[Phillip J. Eby]

At 10:59 AM 1/11/05 +0100, Alex Martelli wrote:
>all you have to do is ADD
>     def __conform__(self, protocol):
>         if issubclass(protocol, Abstract):
>             raise LiskovViolation
>
>that's all.

That will raise a TypeError if protocol is not a class or type, so this 
could probably serve as an example of how difficult it is to write a good 
Liskov-violating __conform__.  :)

Actually, there's another problem with it; if you do this:

     class Y(X): pass
     class Z(Y): pass

then 'adapt(Z(),Y)' will now fail because of a Liskov violation.  It should 
really check for 'protocol is Abstract' or 'protocol in (Abstract,..)' in 
order to avoid this issue.


>Basically, rephrasing private inheritance with containment and delegation 
>is a lot of messy work, and results in far more complicated 
>structures.  And instead of paying the tiny price of a __conform__ call at 
>adaptation time, you pay the price of delegating calls over and over at 
>each x.tm1() call, so it's unlikely performance will improve.

Well, as I mentioned in my other post, such inheritance is a lot simpler 
with PEAK, so I've probably forgotten how hard it is if you're not using 
PEAK.  :)  PEAK also caches the delegated methods in the instance's 
__dict__, so there's virtually no performance penalty after the first access.

Again, not an argument that others should use PEAK, just an explanation as 
to why I missed this point; I've been using PEAK's delegation features for 
quite some time and so tend to think of delegation as something relatively 
trivial.