pantsbuild / pants / 23805124382

93%

Build Type

push

github

Committed by web-flow

Commit Message

An experimental uv-based PEX builder (#23197)

Add an experimental `[python].pex_builder` option that allows using
[uv](https://github.com/astral-sh/uv) to install dependencies when
building PEX files via `pants package`.

When set to `"uv"`, Pants:
1. Downloads the `uv` binary (as an `ExternalTool`).
2. Creates a virtual environment with `uv venv`.
3. Installs dependencies with `uv pip install`.
4. Passes the pre-populated venv to PEX via `--venv-repository`.

When a PEX-native lockfile is available, uv installs the exact pinned
versions with `--no-deps`, preserving reproducibility. Otherwise it
falls back to transitive resolution from requirement strings.

Builds that cannot use uv (internal-only, cross-platform, no local
interpreter) silently fall back to the default pip path.

## Benchmark

Raw `pip install` vs `uv pip install` for 11 packages (requests, boto3,
cryptography, aiohttp, sqlalchemy, pillow, etc.) measured with
[hyperfine](https://github.com/sharkdp/hyperfine):

| Condition | pip | uv | Speedup |
|-----------|-----|-----|---------|
| Cold cache | 6.3s | 4.1s | **1.6x faster** |
| Warm cache | 6.9s | 0.14s | **51x faster** |

Within Pants, the end-to-end improvement is smaller because scheduler
and bootstrap overhead dominates, but the dependency installation step
itself is significantly faster — especially with warm caches on
repeated builds.

## LLM Disclosure

Code was written by the author. Claude was used for code review,
catching edge cases, and verifying test coverage.

Run Details

122 of 138 new or added lines in 5 files covered. (88.41%)

9 existing lines in 1 file now uncovered.

91642 of 98643 relevant lines covered (92.9%)

4.05 hits per line