PEP or other official document about (mandatory) use of "requires_python" and "classifier" (with Python version) fields?

[buhtz]

I know two ways to specify the required Python version.

[project]
# ...
requires-python = ">=3.8"
# ...
classifiers = [
    "Programming Language :: Python :: 3.8"
# ...

This is redundant. But for example tools like pyroma make them mandatory.
I couldn't find a PEP or other official source to decide if I can leave one of that fields?

Do you have any source or opinion about it?

[CAM-Gerlach]

Briefly, classifiers are particularly important to specify the upper bound that the package is actually tested to work against, without being a hard cap for the solver like requires_python (which causes a bunch of problems in practice). See this discussion on the official Packaging Discourse for more information and context there.

However, this would likely be more on topic for the official Python Packaging Discourse instead, since this is spec/usage advice and not anything specific to the implementation in any one particular backend (like Setuptools).

[abravalheri]

They are not mandatory. The only metadata that is mandatory is name (and somehow version, but version can be dynamic). Reference: https://packaging.python.org/en/latest/specifications/declaring-project-metadata/.

Remark: I personally think requires-python is preferred over classifiers because tools are more likely to enforce requires-python.