[Python-Dev] C-level duck typing

["Martin v. Löwis"]

> Agreed in general, but in this case, it's really not that easy. A C
> function call involves a certain overhead all by itself, so calling into
> the C-API multiple times may be substantially more costly than, say,
> calling through a function pointer once and then running over a returned C
> array comparing numbers. And definitely way more costly than running over
> an array that the type struct points to directly. We are not talking about
> hundreds of entries here, just a few. A linear scan in 64 bit steps over
> something like a hundred bytes in the L1 cache should hardly be measurable.

I give up, then. I fail to understand the problem. Apparently, you want
to do something with the value you get from this lookup operation, but
that something won't involve function calls (or else the function call
overhead for the lookup wouldn't be relevant).

I still think this is out of scope for python-dev. If this is something
you want to be able to do for Python 2.4 as well, then you don't need
any change to Python - you can do whatever you come up with for all
Python versions, no need to (or point in) changing Python 3.4 (say).

As this is apparently only relevant to speed fanatics, too, I suggest 
that you check how fast PyPI works for you. Supposedly, they have very
efficient lookup procedures, supported by the JIT. If this doesn't work
for some reason, I suggest that you'll have to trade speed for
convenience: a compile-time fixed layout will beat any dynamic lookup
any time. Just define a common base class, and have all interesting
types inherit from it.

Regards,
Martin