[Python-Dev] advice needed: best approach to enabling "metamodules"?

[Nathaniel Smith]

On Mon, Dec 1, 2014 at 12:59 AM, Nathaniel Smith <njs at pobox.com> wrote:
> On Sun, Nov 30, 2014 at 10:14 PM, Mark Shannon <mark at hotpy.org> wrote:
>> Hi,
>>
>> This discussion has been going on for a while, but no one has questioned the
>> basic premise. Does this needs any change to the language or interpreter?
>>
>> I believe it does not. I'm modified your original metamodule.py to not use
>> ctypes and support reloading:
>> https://gist.github.com/markshannon/1868e7e6115d70ce6e76
>
> Interesting approach!
>
> As written, your code will blow up on any python < 3.4, because when
> old_module gets deallocated it'll wipe the module dict clean. And I
> guess even on >=3.4, this might still happen if old_module somehow
> manages to get itself into a reference loop before getting
> deallocated. (Hopefully not, but what a nightmare to debug if it did.)
> However, both of these issues can be fixed by stashing a reference to
> old_module somewhere in new_module.
>
> The __class__ = ModuleType trick is super-clever but makes me
> irrationally uncomfortable. I know that this is documented as a valid
> method of fooling isinstance(), but I didn't know that until your
> yesterday, and the idea of objects where type(foo) is not
> foo.__class__ strikes me as somewhat blasphemous. Maybe this is all
> fine though.
>
> The pseudo-module objects generated this way will still won't pass
> PyModule_Check, so in theory this could produce behavioural
> differences. I can't name any specific places where this will break
> things, though. From a quick skim of the CPython source, a few
> observations: It means the PyModule_* API functions won't work (e.g.
> PyModule_GetDict); maybe these aren't used enough to matter. It looks
> like the __reduce__ methods on "method objects"
> (Objects/methodobject.c) have a special check for ->m_self being a
> module object, and won't pickle correctly if ->m_self ends up pointing
> to one of these pseudo-modules. I have no idea how one ends up with a
> method whose ->m_self points to a module object, though -- maybe it
> never actually happens. PyImport_Cleanup treats module objects
> differently from non-module objects during shutdown.

Actually, there is one showstopper here -- in the first version where
reload() uses isinstance() is actually 3.4. Before that you need a
real module subtype for reload to work. But this is in principle
workaroundable by using subclassing + ctypes on old versions of python
and the __class__ = hack on new versions.

> I guess it also has the mild limitation that it doesn't work with
> extension modules, but eh. Mostly I'd be nervous about the two points
> above.
>
> -n
>
> --
> Nathaniel J. Smith
> Postdoctoral researcher - Informatics - University of Edinburgh
> http://vorpus.org



-- 
Nathaniel J. Smith
Postdoctoral researcher - Informatics - University of Edinburgh
http://vorpus.org