fix: honor _fresh installs with the uv installer

[r3wretrhy]

Description

--installer uv ignored _fresh=True. Pip maps that flag to --ignore-installed so the CLI and project_wheel_metadata reinstall build requirements when PYTHONPATH is preset. The uv backend accepted _fresh but never forwarded it.

Pass uv pip install --reinstall in that case, matching pip's behavior.

Changelog

• Added changelog fragment: docs/changelog/1179.bugfix.rst

Checklist

• Tests pass locally (uv run pytest tests/test_env.py::test_uv_impl_install_cmd_well_formed tests/test_env.py::test_default_impl_install_cmd_well_formed — 24 passed)
• Code follows project style (ruff check / ruff format --check)

[layday]

Please provide evidence that uv is impacted by the same issue.

[henryiii]

FYI, I was suspicious too that this was valid, and ran a check with Claude:

🤖 AI text below 🤖

No, the PR fixes a bug that does not exist, and the two flags are not equivalent.

The premise is wrong. The reason pip needs --ignore-installed is that pip runs inside the target interpreter and sees packages on PYTHONPATH as "already satisfied". uv is an external binary that inspects the venv's site-packages directly, so it never sees PYTHONPATH packages. On top of that, the uv backend already strips PYTHONPATH from the subprocess environment at src/build/env.py:486. I verified this empirically with uv 0.12.10: with PYTHONPATH pointing at a directory containing packaging, uv pip install packaging into an empty venv installs it anyway, while plain pip install skips it and pip install --ignore-installed installs it. So the uv backend already behaves correctly without this change.

The flags differ in semantics:

• --ignore-installed (pip) does not uninstall anything. It overwrites files on top of whatever is there and can leave stale files behind.
• --reinstall (uv) uninstalls and reinstalls every package, and also implies --refresh, which forces revalidation of cached index metadata against the server. That is extra network traffic on every isolated build for no benefit, since the venv is always freshly created.

Recommendation: close the PR or ask for a reproducer. If a reproducer surfaces, the right uv analogue would be to look at why PYTHONPATH stripping did not work, not to add --reinstall.

[r3wretrhy]

I don't have a uv reproducer. _UvBackend.install_dependencies already drops PYTHONPATH from the subprocess environment, so uv does not see host packages the way in-process pip does. --reinstall is also not equivalent to pip's --ignore-installed (it uninstalls and implies --refresh). Closing.