py4uv

A portable py launcher shim, backed by uv — no Python installation needed. py -3.10 …, py -3.12 … etc. work transparently on Windows and GNU/Linux; uv downloads missing interpreters on demand. (The Unix code is not specific to Linux, so other Unix systems such as macOS may work too, but they are not tested.)

What it does

Translates

py [-3 | -3.12 | -3.12-64 | -V:3.12] <python args...>

into

uv run --no-project [-p <version>] python <python args...>

On Windows the argument tail is taken from the raw command line (GetCommandLineW) and appended to uv's command line verbatim (raw_arg), so quoting, !, %, ^ etc. survive exactly as typed; Ctrl+C is forwarded to Python (the REPL gets KeyboardInterrupt instead of the shim dying) and exit codes propagate. On Unix argv is passed through untouched and uv is exec'd in place of the shim, so signals and exit codes need no forwarding at all.

Version resolution order mirrors the real py launcher:

1. explicit flag (-3, -3.12, -3.12-64, -V:3.12)
2. shebang line of the script given as first argument (#!/usr/bin/env python3.10); interpreter options trailing it (#!/usr/bin/python3 -u) are passed on to python
3. PY_PYTHON environment variable
4. uv's default (UV_PYTHON / .python-version / latest managed install)

A bare major version from any of these (py -3, #!/usr/bin/python3, PY_PYTHON=3) is refined through PY_PYTHON3/PY_PYTHON2, like the real launcher — except from an explicit -V:3 tag, which the real launcher treats as exact.

Version requests beyond X[.Y] work too: Company/version tags (-V:PyPy/3.10, PY_PYTHON=PyPy/3.10) map to uv's implementation@version form (pypy@3.10; PythonCore/ means plain CPython, and an unknown company fails in uv rather than silently running the wrong interpreter), and any other request py4uv doesn't recognize (3.13t, pypy3.10, an interpreter path) is handed to uv run -p verbatim, so uv's own resolution applies.

py -0 / -0p / --list / --list-paths as the first argument map to uv python list --only-installed; anything after the flag is ignored, like the real launcher.

Deliberate divergences from the real launcher: architecture suffixes (-32/-64/-arm64) are accepted but ignored, py -h shows Python's help rather than launcher help, py script.py runs the script with plain python (no PEP 723 inline-dependency handling — use uv run script.py for that), and the PYLAUNCHER_* environment variables (PYLAUNCHER_DRYRUN, PYLAUNCHER_DEBUG, PYLAUNCHER_ALLOW_INSTALL, …) are ignored. Shebangs only ever select a uv-managed interpreter: an absolute-path python shebang (#!C:\proj\venv\Scripts\python.exe) picks uv's python rather than running that file (the real launcher executes the named interpreter, e.g. a venv's), #!/usr/bin/env python3 does not search PATH for a python3 to run, non-python shebang commands are ignored entirely, and the shebang line must be UTF-8 (no ANSI-code-page fallback). On Windows, uv must be a real uv.exe — found on PATH, or failing that next to py.exe — since a .cmd/.bat shim would have cmd.exe re-parse the verbatim argument tail.

Install

• Prebuilt binary: download the archive for GNU/Linux (x86_64, static, so it runs on any distribution) or Windows (x86_64) from the releases and put py (py.exe) on your PATH.
• With uv (PyPI package py4uv, which contains the same binary): uv tool install py4uv puts py next to uv's other tools; without installing: uvx --from py4uv py -3.12 ….
• With cargo: cargo binstall py4uv (downloads the prebuilt binary) or cargo install --locked py4uv (builds it, needs Rust 1.86 or newer).

Build

With Rust 1.86 or newer, straight from GitHub:

cargo install --locked --git https://github.com/sbellon/py4uv

or from a local checkout:

cargo install --locked --path .

Either installs py into ~/.cargo/bin. Or build it with cargo build --release and put target/release/py (py.exe on Windows) on your PATH yourself.

Checks

The typical Rust flow must stay clean (clippy runs with the pedantic group enabled via [lints.clippy] in Cargo.toml):

cargo fmt --check
cargo check --all-targets --all-features
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all-features
cargo doc --no-deps --all-features

plus a clippy pass for the other platform's code via a cross target (x86_64-pc-windows-msvc from GNU/Linux, x86_64-unknown-linux-musl from Windows), e.g.:

cargo clippy --all-targets --all-features --target x86_64-pc-windows-msvc -- -D warnings

A pre-commit hook runs all of it automatically; enable it once per clone with:

git config core.hooksPath .githooks

License

MIT, see LICENSE.