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

Steven D'Aprano steve at pearwood.info
Tue Apr 7 02:10:16 CEST 2009

On Tue, 7 Apr 2009 07:27:29 am Guido van Rossum wrote:

> Unfortunately the language reference is not the only thing we have to
> worry about. Unlike languages like C++, where compiler writers have
> the moral right to modify the compiler as long as they stay within
> the weasel-words of the standard, in Python, users' expectations
> carry value. Since the language is inherently not that fast, users
> are not all that focused on performance (if they were, they wouldn't
> be using Python). Unsurprising behavior OTOH is valued tremendously.

Speaking as a user, Python's slowness is *not* a feature. Anything 
reasonable which can increase performance is a Good Thing.

One of the better aspects of Python programming is that (in general) you 
can write code in the most natural way possible, with the least amount 
of scaffolding getting in the way. I'm with Raymond: I think it would 
be sad if "exp = long(mant * 2.0 ** 53)" did the exponentiation in the 
inner-loop. Pre-computing that value outside the loop counts as 
scaffolding, and gets in the way of readability and beauty.

On the other hand, I'm with Guido when he wrote "it is certainly not 
right to choose speed over correctness". This is especially a problem 
for floating point optimizations, and I urge Cesare to be conservative 
in any f.p. optimizations he introduces, including constant folding.

So... +1 on the general principle of constant folding, -0.5 on any such 
optimizations which change the semantics of a f.p. operation. The only 
reason it's -0.5 rather than -1 is that (presumably) anyone who cares 
about floating point correctness already knows to never trust the 

Steven D'Aprano

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