But namespace packages are still useful for what PEP420 envisages and they should be able to have runnable tests.

For instance

 projectX/
   - interfaces/
         - proxyX.py
         - testX.py
 projectY/
   - intefaces/
         - proxyY.py
         - testY.py

Here interfaces is a namespace package and projectX and projectY are kept separate, perhaps to reduce dependency and/or for separation of concerns.

I don't think this is insurmountable - even taking into account Inada's very good point about dangerous_scripts. A full tests/ packages in on both projectX and projectY would allow their test to run. However, in some environments, it is normal to put the test alongside the code as shown here.

But some better documentation on this issue would advisable, and some guidance on the best practice.

Python has a complex ecosystem, and so an individual developer might hit this problem but be using a third party `framework`, which hasn't taken all of this on board. Eg. https://code.djangoproject.com/ticket/30403 . (I'm not really singling django out, although it's the one I hit) So it important that there is a clear understanding of the best way to deal with this case.