Thank you all for the feedback. I've now updated the PEP to specify a
4-word pyc header with a bit field in every case.

On Fri, Sep 8, 2017, at 09:43, Nick Coghlan wrote:
> On 8 September 2017 at 07:55, Antoine Pitrou <solipsis@pitrou.net> wrote:
> > On Fri, 8 Sep 2017 07:49:46 -0700
> > Nick Coghlan <ncoghlan@gmail.com> wrote:
> >> > 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.
>
> Yeah, we (me, Benjamin, Greg) discussed that here, and we agree -
> there isn't actually any benefit to keeping the timestamp based pyc's
> using the same layout, since the magic number is already going to
> change anyway.
>
> Given that, I think your suggested 16 byte header layout would be a
> good one: 4 byte magic number, 4 bytes reserved for format flags, 8
> bytes with an interpretation that depends on the format flags.
>