[BUG] Using attr for version specification with namespace packages

[hsorby]

setuptools version

setuptools == 58.5.3

Python version

Python 3.9

OS

macOS

Additional environment information

No response

Description

When I try to pip install <namespace-package> using attr to determine the value of version I get a failure message.

Expected behavior

The pip install <namespace-package> command works successfully.

How to Reproduce

I have created an example to show this behaviour:

1. Clone repo. https://github.com/hsorby/setuptools-issue
2. Create virtual environment: python -m venv issue
3. Activate: source issue/bin/activate
4. cd setuptools-issue
5. Install animals.cat package: pip install animals.cat/
6. Install animals.dog package: pip install animals.dog/

This will fail because ModuleNotFoundError: No module named 'animals.dog'

If I now uninstall animals.cat: pip uninstall animals.cat and then repeat steps 5. and 6. they will succeed, I'm guessing because something has been leftover from the uninstall?

Output

    File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
    File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
    File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
  ModuleNotFoundError: No module named 'animals.dog'
  ----------------------------------------

Code of Conduct

• I agree to follow the PSF Code of Conduct

[jaraco]

Thanks for the report.

Is there a reason you're using pkg_resources-style namespace packages? I would not recommend using those except for legacy namespaces, and even for those, I'd recommend to migrate to pkgutil-style (for Python 3.2 or earlier) or PEP 420 native namespace packages.

I suspect the issue is that the attr functionality is relying on simple package resolution and hasn't bootstrapped the namespace packages support. I suspect it will be difficult to correct the issue and for little value with a deprecated feature.

I'd recommend to simply avoid using attr until you're able to migrate away from pkg_resources-style namespace packages.

I welcome you to investigate further to trace the root cause and I'd welcome a patch if it doesn't add too much complication to the implementation.