[Python-Dev] PEP 552: deterministic pycs

[Antoine Pitrou]

On Fri, 8 Sep 2017 07:49:46 -0700
Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 8 September 2017 at 03:04, Antoine Pitrou <solipsis at pitrou.net> wrote:
> > On Thu, 7 Sep 2017 18:47:20 -0700
> > Nick Coghlan <ncoghlan at gmail.com> wrote:  
> >> However, I do wonder whether we could encode *all* the mode settings
> >> into the magic number, such that we did something like reserving the
> >> top 3 bits for format flags:
> >>
> >> * number & 0x1FFF -> the traditional magic number
> >> * number & 0x8000 -> timestamp or hash?
> >> * number & 0x4000 -> checked or not?
> >> * number & 0x2000 -> reserved for future format changes  
> >
> > I'd rather a single magic number and a separate bitfield that tells
> > what the header encodes exactly.  We don't *have* to fight for a tiny
> > size reduction of pyc files.  
> 
> One of Benjamin's goals was for the existing timestamp-based pyc
> format to remain completely unchanged, so we need some kind of marker
> in the magic number to indicate whether the file is using the new
> format or nor.

I don't think that's a useful goal, as long as we bump the magic number.

Note the header format was already changed in the past when we added a
"size" field beside the "timestamp" field, to resolve collisions due to
timestamp granularity.

Regards

Antoine.