Warn about redundant license field if classifier is present

[merwok]

Projects are supposed to use a license classifier, if one exists for their license.
The license field is meant to be free-form text to specify a non-standard license or to add details to a classifier (to specify Other :: Proprietary, or maybe to add patent note for example).
Many projects needlessly define license, flit could warn about that when it validates metadata.

[takluyver]

I've also noticed that a lot of people set both, or prefer the license field. But at some point, it seems likely that it will be possible to express richer & more detailed license information in the license field as SPDX expressions (PEP 639). So I don't want to push people too much towards classifiers for license information, when we might be pushing them in another direction in a few months.

[Conchylicultor]

A similar point could be made for the requires-python field.

With poetry, "Programming Language :: Python :: 3", and "License :: OSI Approved :: Apache Software License" classifiers are automatically set.

Maybe Flit could do the same ?

This would push people to use license= and requires-python fields rather than classifiers.

[pradyunsg]

I don't think we should add such a warning for license. Rather, I think that we should not do anything here right now. See pypa/trove-classifiers#17, which is about how existing license classifiers are not sufficeint.

Once PEP 639 is accepted, we'll deprecate both the license classifiers as well as the regular table-based license fields. Basically, we're going to get license = "MIT" once that happens and will want to push users to adopt that.

[merwok]

Let’s see where the PEP goes.