fix namespace package handling

[RomanMatthew]

easy_install and pip have conflicting way of handling namespaced packages (diffpy functions as a namespace only and is not a real package) and a mixed installation then may not work:

$ conda create -n xx36 -c diffpy diffpy.utils
$ conda activate xx36
$ python -c 'import diffpy.utils' # OK
$ pip install https://github.com/diffpy/diffpy.pdfmorph/archive/master.tar.gz
$ python -c 'import diffpy.utils' # FAILS
Traceback (most recent call last):
File "", line 1, in
ModuleNotFoundError: No module named 'diffpy.utils'
This actually happens because of an outdated definition of namespace package in diffpy/init.py which is specific to setuptools (easy_install) and does not work well with pip. Other diffpy packages have that code updated, e.g., diffpy.structure/src/diffpy/init.py, and then it seems to work with both easy_install and pip.

fix namespace package handling in diffpy/init.py as in diffpy.structure and
check if mixed easy_install, pip installation of several diffpy packages works OK
and if it does, update README to suggest pip as the installation tool

[chloeann95]

I'm unsure what this wants to do and also not sure how to fix. If we tell them to install with pip and it works properly by doing so do we need to plan for mixed installations? Even with this change we're suggesting they use pip to install anyways.

[sbillinge]

I am not sure either. It is well documented by Roman and may therefore have been fixed if it works. But the final check I would do if I took it on would be to look and see what I see in diffpy/init.py in diffpy.utils and compare it with the same file in diffpy.structure. Also to look back through the commit log to see if a fix was merged at some point in the past. I will take a look when I get a chance, but this would be my feedback.

[Sparks29032]

The diffpy/__init__.py files in diffpy.utils and diffpy.structure have been the same since 2017. Running all five of the below command lines did not result in any errors on git-bash.

$ conda create -n xx36 -c diffpy diffpy.utils
$ conda activate xx36
$ python -c 'import diffpy.utils'
$ pip install https://github.com/diffpy/diffpy.pdfmorph/archive/master.tar.gz
$ python -c 'import diffpy.utils'

No error results from running only the two lines below either.

$ pip install https://github.com/diffpy/diffpy.utils/archive/refs/tags/v3.1.0.tar.gz
$ python -c 'import diffpy.utils'

I believe the outdated definition specific to setuptools was removed in diffpy/diffpy.utils@3105569. I am speculating that this issue occurs from using the v1.2.2 release of diffpy.utils rather than the v3+ releases.

[sbillinge]

Thanks @Sparks29032. Closing as resolved