[Import-SIG] [Distutils] nspkg.pth files break $PYTHONPATH overrides

[PJ Eby]

On Mon, Mar 24, 2014 at 6:09 PM, Barry Warsaw <barry at python.org> wrote:

> On Mar 24, 2014, at 05:53 PM, Donald Stufft wrote:
>
> >See also https://github.com/pypa/pip/issues/3
>
> Hah, yeah.  I didn't realize --single-version-externally-managed is
> implied by
> --root, and in fact our Debian build scripts often (either explicitly or
> implicitly) define --root, as is the case in lazr.uri.
>
> >Basically prior to PEP420 namespace packages were bad and using them
> results
> >in pain sooner or later :( I'm not sure if a good solution yet, perhaps we
> >can backport PEP420 to PyPI and have namespace packages depend on that?
>
> Or maybe do a version check before installing this file?  Python 3.3 and
> newer
> will have PEP 420 support, and this file makes no sense in that case.
>

That won't *quite* work, because the .pth files are generated for other
types of direct-install distributions.

I think the correct fix would be to change the nspkg.pth magic to check for
PEP 420 support, but unfortunately it seems we may have to use version
checking: on rereading PEP 420 I see there's no 'sys.namespace_packages' or
similar object that can be directly checked for feature support.  :-(
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