[Import-SIG] PEP 489: Multi-phase extension module initialization; version 5

Nick Coghlan ncoghlan at gmail.com
Thu May 21 00:16:54 CEST 2015


On 21 May 2015 at 00:56, Eric Snow <ericsnowcurrently at gmail.com> wrote:
> On Wed, May 20, 2015 at 4:55 AM, Petr Viktorin <encukou at gmail.com> wrote:
>> The point is that exec_module doesn't a priori depend on the module
>> being in sys.modules, which I think is a good thing.
>
> Well, there's an explicit specification about how sys.modules is used
> during loading.  For post-exec sys.modules lookup specifically,
> https://docs.python.org/3.5//reference/import.html#id2.  The note in
> the language reference says that it is an implementation detail.
> However, keep in mind that this PEP is a CPython-specific proposal.
>
> That said, I'm only -0 on not matching the sys.modules lookup behavior
> of module loading.  It could be okay if we were to document the
> behavior clearly.  My concern is with having different semantics even
> if it only relates to a remote corner case.  It may be a corner case
> that someone will rely on.

We *will* match the semantics for the *overall* loading process. What
Petr is saying is that *while* executing the "execution slots",
they'll all receive the object returned by Py_mod_create (or the
automatically created module if that slot is not defined), rather than
any replacement injected into sys.modules.

There's no Python level parallel for that "multiple execution slots"
behaviour, so it makes sense to define the semantics based on
simplicity of implementaiton and the fact we want to encourage the use
of Py_mod_create for extension modules over sys.modules injection.

>> No other implementation of exec_module checks sys.modules in the middle
>> of its operation. So I think the semantics are consistent.
>
> I was thinking of each exec slot as a parallel to Loader.exec_module.
> Thus I was expecting the same sys.modules lookup behavior that you get
> during module loading.  That's why I would expect the module to get
> updated to sys.modules[spec.name] after each exec slot runs.

I changed my mind when Petr posted the clarification that this is
really just a matter of iterating over the defined slots in the
loader's exec_module method, and calling any of them that are defined
as execution slots (for the time, just Py_mod_exec).

The entirety of a Python module runs in the same module namespace,
regardless of what is done with sys.modules, so having all execution
slots called with the same object is the extension module equivalent.

Cheers,
Nick.

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


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