[Python-ideas] Implicit submodule imports

[Brett Cannon]

On Sat Sep 27 2014 at 11:37:16 AM Nathaniel Smith <njs at pobox.com> wrote:

> On Sat, Sep 27, 2014 at 1:33 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> > Or perhaps these special "modules" could subclass ModuleType and somehow
> > get reloading to work correctly. In 2.7 at least you can manually copy a
> > module to a module subclass, install it into sys.modules, and reload
> > will accept it. Not only that, but after reloading it still uses the
> > same subclass.
> >
> > Unfortunately, when I tried it in 3.3, imp.reload complained about my
> > custom module subclass not being a module, so it seems that 3.3 at least
> > is more restrictive than 2.7. (Perhaps 3.3 reload does a "type(obj) is
> > ModuleType" instead of isinstance test?)
>
> Yeah, it looks like 3.3 does an explicit 'type(obj) is ModuleType'
> check, but is the only version that works like this -- earlier and
> later versions both use isinstance.
>

Feel free to file an issue about this.

-Brett


>
> > Nevertheless, I got this proof of concept more-or-less working in 2.7
> > and 3.3:
> >
> > import sys
> > from types import ModuleType
> >
> > class MagicModule(ModuleType):
> >     def __getattr__(self, name):
> >         if name == "spam":
> >             return "Spam spam spam!"
> >         raise AttributeError
> >
> > eggs = 23
> >
> > _tmp = MagicModule(__name__)
> > _tmp.__dict__.update(sys.modules[__name__].__dict__)
> > sys.modules[__name__] = _tmp
> > del _tmp
>
> This approach won't work well for packages -- imagine that instead of
> 'eggs = 23', the body of the file imports a bunch of submodules. If
> those submodules then import the top-level package in turn, then
> they'll end up with the original module object and namespace, not the
> modified one.
>
> One could move the sys.modules assignment up to the top of the file,
> but you can't move the __dict__.update call up to the top of the file,
> because you can't copy the old namespace until after it's finished
> being initialized. OTOH leaving the __dict__.update at the bottom of
> the file is pretty risky too, because then any submodule that imports
> the top-level package will see a weird inconsistent view of it until
> after the import has finished.
>
>  The solution is, instead of having two dicts and updating one to
> match the other, simply point the new module directly at the existing
> namespace dict, so they always stay in sync:
>
>   _tmp = MagicModule(__name__)
>   _tmp.__dict__ = sys.modules[__name__].__dict__
>
> ...except this gives an error because module objects disallow
> assignment to __dict__.
>
> Sooooo you're kinda doomed no matter what you do.
>
> --
> Nathaniel J. Smith
> Postdoctoral researcher - Informatics - University of Edinburgh
> http://vorpus.org
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