On Sat., Jun. 5, 2021, 07:38 Robin Becker, <ro...@reportlab.com> wrote:
> On 04/06/2021 19:29, Steve Dower wrote:
> ...........
> >
> > While we were investigating the reliably broken behaviour, we figured
> that related behaviour was
> > *unreliably* broken on all platforms.
> >
> > Specifically, if you imported a module through a relative path (resolved
> to CWD), changed the CWD, cleared the module
> > cache, and reimported, you would still get the original module, contrary
> to the intent of getting the newly resolved
> > module. ("Correct" code would have also cleared the import path caches,
> not just sys.modules.) This was because the
> > module info was cached using the relative path, and so would be used
> later even though its absolute path had changed.
> >
> > Someone reported this change in 3.8 and we decided to revert it in
> existing released, because the bugfix was for an
> > obscure enough situation that it really wasn't worth breaking any
> existing versions. For unreleased versions, it's
> > better to fix it so that imports will be more reliable in the future.
>
>
> Unfortunately we are using these relative imports to implement plugins for
> an xml language processor so this is likely
> not just an accident of ordering the failing tests. I agree we have to get
> a plugin module a bit more directly using the
> import internals. I still think we might need to control sys.path as
> presumably the plugin code could have imports of
> parallel modules.
>
> I'm beginning to understand that messing about with
> sys.path/modules/meta_path etc etc is a lot harder than it used to be.
>
How so? The key details have not changed since Python 3.4 when module specs
were introduced. And meta path, etc. are 2.7 concepts.
> Incidentally I find that I am able to import a namespace module directly;
> this code doesn't crash (at least in python >= 3.6
>
> ###################################################
> #timpA.py don't run with any meaningful directory A
> import importlib
> def main():
> module = importlib.import_module('A')
> print('module=%r' % module)
>
> if __name__=='__main__':
> main()
> ###################################################
>
> in python 3.6-3.9 this prints out "module=<module 'A' (namespace)>" in
> 3.10.0b1 it prints
> "module=<module 'A' (<_frozen_importlib_external._NamespaceLoader object
> at 0x7f1c8a22fe20>)>".
> I thought I had to use pkgutils or pkg_resources to mess with these
> new-fangled packages.
>
Nope, that has never been required since the concept of namespace packages
were introduced to import.
> Likewise I find that the python statement 'import A' also doesn't fail and
> A is a namespace.
Yep, that's expected.
I feel quite stupid now :(
>
As soon as you step outside the norm there are a lot of details to
understand; import is very much following the "Complex is better than
complicated" zen. So no need to feel stupid, there's a lot to keep track of
when you start to dive into the details.
-Brett
--
> Robin Becker
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