Python's native-inputs

[Nicolas Graves]

Hi Guix,

On some languages, there are a lot of unused native-inputs that are
development & linting dependencies much more than packages that are
actually used to build or test a package (I'm thinking at least Python
and Rust). These fall in the category of tools "useful" at run time, but
unecessary at build time.

Is there a clear policy about their removal? I've seen the case of
pre-commit in Python, and I've commited a series yesterday regarding
pylint, but there are a whole lot of them in Python, at least :

- black and extensions
- codecov
- coveralls
- coverage
- pytest-cov
- pycodestyle
- mypy and extensions
- isort and extensions
- flake8 and extensions
- possibly virtualenv and extensions
and I'm probably forgetting some.

These packages make a lot of sense when considering things like
`guix shell -D` but they are hampering some progress on Python packages
since they are everywhere and a small update in their inputs rebuilds
the whole python world (even though it has NO influence on the
functionality of any other package).

What are the guidelines in this case?

I can propose a huge patch series (currently ~300 patches, and not
finished), to remove them, lint against them and remove them from the
importer as a default, but that's a big decision to make. IMO we should
have a dev-inputs field to handle these cases, but that's even more work.

-- 
Best regards,
Nicolas Graves