Purpose of .python-version?

[denkasyanov]

Currently on uv init a project boilerplate created includes the file .python-version.

What is the purpose of this file, if there are constraints in pyproject.toml and uv.lock?

Is it just for compatibility with tools like pyenv or is there more to it?

I really like how clean project directory became after moving a lot of stuff (like requirements.*) to pyproject.toml. So wonder, if .python-version is really required in two senses:

1. Is it required for the projects now? I tested by deleting .python-version, and everything works as expected and this file is not recreated with commands like uv run, uv sync and uv lock
2. Since a lot of constraints/pinned versions of things are now in pyproject.toml and uv.lock, maybe if .python-version still plays some important role, its function can be moved to pyproject.toml or uv.lock in future?

Asking as a 5-year user of pyenv and .python-version. They are great, but I don't really miss them.

Also, couldn't find peps related to this file.

[my1e5]

Related - #8247

A .python-version file is not strictly required, but it's useful to have it when developing a project as it allows you to specify the exact Python version you are using to do development. It's different to the requires-python field - that is the range of Python versions supported by your project.

[denkasyanov]

Thanks!

It matches my intuition and #8247 indeed provides useful context!