[Python-Dev] PEP 246, redux

[Phillip J. Eby]

At 05:02 PM 1/12/05 +0100, Alex Martelli wrote:
>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)].

One question, and one suggestion.

The question: should the registry support explicitly declaring that a 
particular adaptation should *not* be used, thus pre-empting later phases 
entirely?  This would allow for the possibility of speeding lookups by 
caching, as well as the option to "opt out" of specific adaptations, which 
some folks seem to want.  ;)

The suggestion: rather than checking isinstance() in adapt(), define 
object.__conform__ such that it does the isinstance() check.  Then, Liskov 
violation is simply a matter of returning 'None' from __conform__ instead 
of raising a special error.


>   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.

Okay, one more suggestion/idea:

$ timeit -s "d={}; d[1,2]=None" "d[1,2]"
1000000 loops, best of 3: 1.65 usec per loop

$ timeit -s "d={}; d[1]={2:None}" "d[1][2]"
1000000 loops, best of 3: 0.798 usec per loop

This seems to suggest that using nested dictionaries could be faster under 
some circumstances than creating the two-tuple to do the lookup.  Of 
course, these are trivially-sized dictionaries and this is also measuring 
Python bytecode speed, not what would happen in C.  But it suggests that 
more investigation might be in order.