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

Mark Dickinson dickinsm at gmail.com
Mon Apr 6 22:22:28 CEST 2009

On Mon, Apr 6, 2009 at 9:05 PM, Raymond Hettinger <python at rcn.com> wrote:
> The code for the lsum() recipe is more readable with a line like:
>  exp = long(mant * 2.0 ** 53)
> than with
>  exp = long(mant * 9007199254740992.0)
> It would be ashamed if code written like the former suddenly
> started doing the exponentation in the inner-loop or if the code
> got rewritten by hand as shown.

Well, I'd say that the obvious solution here is to compute
the constant 2.0**53 just once, somewhere outside the
inner loop.  In any case, that value would probably be better
written as 2.0**DBL_MANT_DIG (or something similar).

As Antoine reported, the constant-folding caused quite
a confusing bug report (issue #5593):  the problem (when
we eventually tracked it down) was that the folded
constant was in a .pyc file, and so wasn't updated when
the compiler flags changed.


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