Use SPDX license identifier

[cdce8p]

PEP 639 recommends the use of SPDX license identifiers.

At the moment the license key is ignored by flit, see pypa/flit#525. However, it's still possible to replace it now. Once the support in flit is added, the value will be used automatically.

At some point, the table will need be be changed to just a string for full PEP 639 compliance. That requires support for it first though. The change is also pretty simple.

-license = {text = "BSD-3-Clause"}
+license = "BSD-3-Clause"

[cdce8p]

Working on adding support in flit now. Marking it as draft until then.

[davidism]

I moved to license = {file = "LICENSE.txt"} because when I re-read the current spec months ago, it seemed like that was "more correct" until SPDX was supported. license = {text = "BSD-3-Clause"} isn't technically valid (it specifies the full text of the license). I'm fairly sure we need to hold off until we can specify both license = "BSD-3-Clause and `license_files = ["LICENSE.txt]" so that both the SPDX and file are present.

[cdce8p]

I moved to license = {file = "LICENSE.txt"} because when I re-read the current spec months ago, it seemed like that was "more correct" until SPDX was supported.

Yeah, that's the current spelling in the docs. I'd just argue that it's probably outdated. There is an open issue to rework that section of the packaging docs. pypa/packaging.python.org#1245
From my experience, it's actually better to use to existing license field to in a way "back-populate" the license expression until that is fully supported

license = {text = "BSD-3-Clause"} isn't technically valid (it specifies the full text of the license).

Just from a metadata perspective, it's fully valid. At least setuptools, poetry, and hatch support it.

I'm fairly sure we need to hold off until we can specify both license = "BSD-3-Clause and `license_files = ["LICENSE.txt]" so that both the SPDX and file are present.

While working on a PR for flit, I noticed that all project.license.file really does atm is include the license file in the sdist. For the wheel a glob ('COPYING*', 'LICENSE*') is used to put in .dist-info. Besides that it should be .dist-info/licenses, the current behavior can also be archived by adding LICENSE.txt to [tool.flit.sdist.include].