[Python-Dev] PEP 451 update

Nick Coghlan ncoghlan at gmail.com
Sat Nov 2 10:46:12 CET 2013

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

> 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".


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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