[Python-Dev] PEP 451 update

Brett Cannon brett at python.org
Sat Nov 2 16:21:39 CET 2013

On Sat, Nov 2, 2013 at 5:46 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 2 November 2013 08:44, Eric Snow <ericsnowcurrently at gmail.com> wrote:
> > On Fri, Nov 1, 2013 at 11:59 AM, Brett Cannon <brett at python.org> wrote:
> >> Thanks for the clarification. It all SGTM.
> >
> > I've updated the PEP and the reference implementation.
> While I was saying loader when I should have been saying finder, Eric
> correctly divined my intent :)
> > If I've missed
> > anything please let me know.  There is only one thing I thought of
> > that I'd like to get an opinion on:
> >
> > In the Finders section, the PEP specifies returning None (or using a
> > different loader) when the found loader does not support loading into
> > an existing module (e.g during reload). An alternative to returning
> > None would be for the finder to raise ImportError with a message like
> > "the loader does not support reloading the module". This may actually
> > be a better approach since "could not find a loader" and "the found
> > loader won't work" are different situations that a single return value
> > (None) may not sufficiently represent.
> Throwing ImportError for "can load, but cannot load into that target
> module" sounds good to me.

SGTM as well.

> We should also be explicit that throwing an
> exception from find_spec *stops* the search (unlike returning None).

I thought that was obvious but more docs doesn't hurt. =)

> > Other than that, I'm not aware of any blockers for the PEP.
> The one slight quibble I have with the PEP update is the wording
> regarding the "existing" parameter. I think runpy should pass the new
> parameter as well when calling find_spec, but in that case it *won't*
> be an existing copy of the named module, it will be
> sys.modules["__main__"]. So we shouldn't set an expectation that the
> existing module passed in will necessarily be a previous copy of the
> module being loaded, it's just an indication to the finder that the
> target namespace for execution will be a pre-existing module rather
> than a new one.
> For the same reason, I also have a mild preference for "target" (or
> the more explicit "load_target") as the name, although I won't object
> if you and Brett prefer "existing".

I would go with "module" or "target" and that is where I shall end the
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