Cryptic error for python -m uv pip compile --python-version x.y.z when using the Windows app store Python app

[ncoghlan]

I am running the PSF's CPython build from the Windows app store in order to benefit from the automatic updates.

This build omits the Python launcher for Windows, so the py --list-paths command isn't available.

This results in a cryptic error when requesting transitive dependency resolution for a Python version other than the one used to invoke uv:

> python -m uv pip compile --python-version 3.11 requirements.in
error: The `py` launcher could not be found.

This arises even if source builds are disabled (so there shouldn't be any need to extract dynamic runtime dependency information from a source build):

> python -m uv pip compile --no-build --python-version 3.11 requirements.in
error: The `py` launcher could not be found.

The actual error in these commands was that the --python command line option to specify the target Python runtime was missing (I had been mislead by testing on Linux, which had used Fedora's python3.11 alternative Python install to satisfy the 3.11 version check - it would actually be handy if there was a --toolchain-preference only-specified option to keep uv from trying to autodiscover Python runtimes).

[charliermarsh]

This arises even if source builds are disabled (so there shouldn't be any need to extract dynamic runtime dependency information from a source build)

FWIW I don't think this part is that relevant -- we need to query the Python interpreter to extract the platform markers regardless.

[charliermarsh]

I'm guessing that should be non-fatal or something but defer to @zanieb who owns Python discovery.

[ncoghlan]

FWIW I don't think this part is that relevant -- we need to query the Python interpreter to extract the platform markers regardless.

That's a fair point, so I think that just becomes a small comment on the README.md text about Python discovery (it gives slightly misleading info on why uv needs a target Python with the right version available)

[zanieb]

We're in the process of rewriting all that documentation. I'll improve that and add more context to that error message, thanks!

[zanieb]

I put up a fix at #4620, let me know if you get a chance to test it.

@ncoghlan Regarding

--toolchain-preference only-specified option to keep uv from trying to autodiscover Python runtimes).

In this case, you'd expect us to error whenever --python wasn't included?