Specify specific or maximum Python version

[domoritz]

I got a bit confused that juv init --python=3.9 notebook.ipynb installs at least Python 3.9 rather than specifically 3.9. Maybe the command should be juv init --python='>=3.9' notebook.ipynb or juv init --min_python=3.9 notebook.ipynb.

I see that this option corresponds to the UV_PYTHON variable so maybe this "works as intended" but at least I wanted to drop a note here if someone else runs into this. I was able to specify a max version in the notebook itself.

# /// script
# requires-python = "<3.10"
# dependencies = [
# ]
# ///

[domoritz]

I think I was holding this wrong. The python version isn't really specified in the notebook but instead in the run command.

juv run --python=3.9 notebook.ipynb. I'm not sure there is an issue here.

[manzt]

I see that this option corresponds to the UV_PYTHON variable so maybe this "works as intended" but at least I wanted to drop a note here if someone else runs into this.

Indeed, the behavior is inherited from uv (e.g., uv init --python=3.9 --script foo.py), where the semantics set the lower bound of the requires-python field in the metadata.

I think this makes sense for pure Python scripts, as they should be forward-compatible between minors.... however, an issue often arises with binaries or wheels for dependencies, where a new Python release may lack a compatible wheel. In that case, I'd prefer the latest-compatible Python version with a wheel (rather than triggering a build).

I recommend providing the flag with run or manually pinning an upper bound in the inline metadata (as you have shown). More broadly, in my experience the relationship between requested Python and dependencies has a little friction (and potentially different semantics).

• exclude-newer does not influence selection of Python interpreter, should it? astral-sh/uv#9445

[manzt]

I'm going to pin this issue because I don't know the resolution (but I assume there might be some discussion in uv about this same thing for standalone scripts).