This is a follow up of 2 previous discussions in the Python discourse and an issue inpypa/packaging-problems:

• https://discuss.python.org/t/pep-660-and-setuptools/14855/15
• https://discuss.python.org/t/how-to-implement-namespace-packages-as-in-pep-420-via-import-hooks/15192
• Uncertainty on how to implement namespace packages (as in PEP 420) via import hooks pypa/packaging-problems#588

Summary

One of the popular ways of implementing PEP 660 is via import hooks (since other alternatives likesymlinking package folders are not available in all platforms).

However the reference implementationeditables currently does not support namespaces.
Specifically thedynamic path computation aspect of PEP 420 is a challenge.

After inspecting the implementation forimportlib._bootstrap_external, I may have managed to find 2 solutions for this problem, described in the detail inthis discourse post and demonstrated as a PoC inthis gist:

• Solution A: Create a custom "path class" that tries to emulate the_NamespacePath behaviour.
• Solution B: Create a customPathEntryFinder and add a “bogus” entry tosys.path just to trigger it (implicitly relying thatimportlib.machinery.PathFinder will take care of setting__path__ to_NamespacePath object under the hood).

However, the problem is that I don't know if these approaches are reliable in the long run and in accordance with the language specification.

Open Questions

Questions like the following are still not clear to me:

1. Is there a chance that an internal check for namespace packages will fail if__path__ uses a custom class instead of_NamespacePath even if the behaviour described in PEP 420 is observed?
2. Can we rely on the fact that the import machinery will automatically set the namespace loader for specs in the form ofModuleSpec("name", None, is_package=True)?
3. Ispkgutil (and particularlypkgutil.extend_path) still considered first class citizen of the standard library or is it better to avoid it when writing new code?
4. Can we rely on the behaviour ofsys.path_hooks +importlib.machinery.PathFinder to createModuleSpec objects for namespaces that will automatically perform dynamic path computation? Or is this an internal implementation detail that can change in the future?

If the discussed approaches are not recommended, how can we support namespace packages using import hooks and deliver a complete solution for PEP 660?