[Python-Dev] pyc files, constant folding and borderline portability issues

[Cesare Di Mauro]

On Mon, Apr 6, 2009 18:57, skip at pobox.com wrote:
>
>     Cesare> At this time with Python 2.6.1 we have these results:
>     Cesare> def f(): return 1 + 2 * 3 + 4j
>     ...
>     Cesare> def f(): return ['a', ('b', 'c')] * (1 + 2 * 3)
>
> Guido can certainly correct me if I'm wrong, but I believe the main point
> of
> his message was that you aren't going to encounter a lot of code in Python
> which is amenable to traditional constant folding.  For the most part,
> they
> will be assigned to symbolic "constants", which, unlike C preprocessor
> macros aren't really constants at all.  Consequently, the opportunity for
> constant folding is minimal and probably introduces more opportunities for
> bugs than performance improvements.
>
> Skip

I can understand Guido's concern, but you worked as well on constant
folding, and you know that there's space for optimizations here.

peephole.c have some code for unary, binary, and tuple/list folding; they
worked fine. Why mantaining unuseful and dangerous code, otherwise?

I know that bugs can come out doing such optimizations, but Python have a
good tests battery that can help find them. Obviously tests can't give us
100% insurance that everything works as expected, but they are very good
starting point.

Bugs can happen at every change on the code base, but code base changes...

Cesare