[Python-Dev] PEP 552: deterministic pycs

[Benjamin Peterson]

On Thu, Sep 7, 2017, at 14:21, Antoine Pitrou wrote:
> On Thu, 07 Sep 2017 14:08:58 -0700
> Benjamin Peterson <benjamin at python.org> wrote:
> > On Thu, Sep 7, 2017, at 14:00, Antoine Pitrou wrote:
> > > On Thu, 07 Sep 2017 13:39:21 -0700
> > > Benjamin Peterson <benjamin at python.org> wrote:  
> > > > Hello,
> > > > I've written a short PEP about an import extension to allow pycs to be
> > > > more deterministic by optional replacing the timestamp with a hash of
> > > > the source file: https://www.python.org/dev/peps/pep-0552/  
> > > 
> > > Why isn't https://github.com/python/cpython/pull/296 a good enough
> > > solution to this problem?  It has a simple implementation, and requires
> > > neither maintaining two different pyc formats nor reading the entire
> > > source file to check whether the pyc file is up to date.  
> > 
> > The main objection to that model is that it requires modifying source
> > timestamps, which isn't possible for builds on read-only source trees.
> 
> Not sure how common that situation is (certainly the source tree wasn't
> read-only when you checked it out or untar'ed it), but isn't it easily
> circumvented by copying the source tree before building?

Well, yes, in these kind of "batch" build situations, copying is
probably fine. However, I want to be able to have pyc determinism even
when developing. Copying the entire source every time I change something
isn't a nice.

> 
> > This proposal also allows reproducible builds even if the files are
> > being modified in an edit-run-tests cycle.
> 
> I don't follow you here.  Could you elaborate?

If you require source timestamps to be fixed and deterministic, Python
won't notice when a file is modified.

The larger point is that while the SOURCE_EPOCH patch will likely work
for Linux distributions, I'm interested in being able to have
deterministic pycs in "normal" Python development workflows.