The PEP 420 -- Implicit Namespace Packages introduces packages which have no __init__.py file and can be made of multiple packages. It looks like doctest doesn't support them.

I would like to convert the Lib/test/ of the Python standard library into such package for the issue #26295.

I propose to use an heuristic when the package is only made of one directory (when module.__path__ only contains one entry): use this directory to lookup for requested test files.

Attached patch implements that.