[Python-Dev] PEP 451 update

[Nick Coghlan]

On 31 Oct 2013 08:54, "Eric Snow" <ericsnowcurrently at gmail.com> wrote:
>
> On Wed, Oct 30, 2013 at 4:09 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> > On 31 Oct 2013 03:41, "Eric Snow" <ericsnowcurrently at gmail.com> wrote:
> >> Our recent discovery about reloading should probably be reflected in
> >> the signature of finder.find_spec():
> >>
> >>   MetaPathFinder.find_spec(name, path=None, existing=None)
> >>   PathEntryFinder.find_spec(name, existing=None)
> >>
> >> This way the finder has an opportunity to incorporate information from
> >> an existing spec into the spec it returns.  reload() would make use of
> >> this by passing module.__spec__ (or None if the module has no
> >> __spec__) to _bootstrap._find_spec().
> >>
> >> This approach should also address what you are looking for.  I'd
> >> prefer it over passing the existing spec to exec_module().  The module
> >> (and its __spec__) should have everything exec_module() needs to do
> >> its job.
> >
> > Yes, that should work.
>
> Cool.  I'll update the PEP.
>
> >
> >> We would still need to use loader.supports_reload() in reload().
> >
> > Why? If the reload isn't supported, exec_module can just throw an
exception
> > based on the loader state in the spec.
>
> At the point that exec_module() gets called, the loader can't check
> sys.modules to see if it's a reload or not.  As a workaround, the
> finder could set up some loader state to indicate to the loader that
> it's a reload and then the loader, during exec_module(), would check
> that and act accordingly.  However, that's the sort of boilerplate
> that PEP 451 is trying to offload onto the import machinery.  With
> Loader.supports_reload() it's a lot cleaner.

There's also the option of implementing the constraint directly in the
finder, which *does* have the necessary info (with the change to pass the
previous spec to find_spec).

I still think it makes more sense to leave this out for the moment - it's
not at all clear we need the extra method, and adding it later would be a
straightforward protocol update.

Cheers,
Nick.

>
> -eric
>
> >
> > From the import system's point of view "reload not permitted" is no
> > different from any other exec time failure.
> >
> > Cheers,
> > Nick.
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