[Python-Dev] pyc files, constant folding and borderline portability issues (original) (raw)

Cesare Di Mauro cesare.dimauro at a-tono.com
Mon Apr 6 21:23:18 CEST 2009

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

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