uv/README.md

# Puffin

An extremely fast Python package installer and resolver, written in Rust. Designed as a drop-in replacement for `pip` and `pip-compile`.

Puffin is backed by [Astral](https://astral.sh), the creators of [Ruff](https://github.com/astral-sh/ruff).

## Highlights

- ⚡️ 10-100x faster than `pip` and `pip-tools` (`pip-compile` and `pip-sync`).
- 💾 Disk-space efficient, with a global cache for dependency duplication and Copy-on-Write
  installation on supported platforms.
- 🐍 Installable via `pip`, `pipx`, `brew` etc. Puffin is a single static binary that can be
  installed without Rust or even a Python environment.
- 🧪 Tested at-scale against the top 10,000 PyPI packages.
- ⚖️ Drop-in replacement for common `pip`, `pip-tools`, and `virtualenv` commands.
- 🤝 Support for a wide range of advanced `pip` features, including: editable installs, Git
  dependencies, direct URL dependencies, local dependencies, constraints, source distributions,
  HTML and JSON indexes, and more.
- 🧰 Novel resolution features such as override of transitive dependency versions and a lowest
  compatible version resolution strategy.

## Getting Started

Puffin is available as [`puffin`](https://pypi.org/project/puffin/) on PyPI:

```shell
pipx install puffin
```

To create a virtual environment with Puffin:

```shell
puffin venv  # Create a virtual environment at .venv.
```

To install a package into the virtual environment:

```shell
puffin pip install flask                # Install Flask.
puffin pip install -r requirements.txt  # Install from a requirements.txt file.
puffin pip install -e .                 # Install the current project in editable mode.
```

To generate a set of locked dependencies from an input file:

```shell
puffin pip compile pyproject.toml -o requirements.txt   # Read a pyproject.toml file.
puffin pip compile requirements.in -o requirements.txt  # Read a requirements.in file.
```

To install a set of locked dependencies into the virtual environment:

```shell
puffin pip sync requirements.txt  # Install from a requirements.txt file.
```

Puffin's `pip-install` and `pip-compile` commands supports many of the same command-line arguments
as existing tools, including `-r requirements.txt`, `-c constraints.txt`, `-e .` (for editable
installs), `--index-url`, and more.

## Roadmap

Puffin is an extremely fast Python package resolver and installer, designed as a drop-in
replacement for `pip` and `pip-tools` (`pip-compile` and `pip-sync`).

Puffin is not a complete package manager. Instead, it represents an intermediary goal in our
pursuit of a "Cargo for Python": a Python package and project manager that is extremely fast,
reliable, and easy to use — a single tool capable of unifying not only `pip` and `pip-tools`, but
also `pipx`, `virtualenv`, `tox`, `setuptools`, `poetry`, `pyenv`, `rye`, and more.

In the future, Puffin will be used as the foundation for such a tool: a single binary that
bootstraps your Python installation and gives you everything you need to be productive with Python.

In the meantime, though, Puffin's narrower scope allows us to solve many of the low-level problems
that are required to build such a package manager (like package installation) while shipping an
immediately useful tool with a minimal barrier to adoption. Try it today in lieu of `pip` and
`pip-compile`.

## Limitations

Puffin does not yet support Windows ([#73](https://github.com/astral-sh/puffin/issues/73)).

Puffin does not support the entire `pip` feature set. Namely, Puffin won't support the following
`pip` features:

- `.egg` dependencies
- Editable installs for Git and direct URL dependencies (though editable installs _are_ supported
  for local dependencies)
- ...

On the other hand, Puffin plans to (but does not currently) support:

- Hash checking
- ...

Like `pip-compile`, Puffin generates a platform-specific `requirements.txt` file (unlike, e.g.,
`poetry` and `pdm`, which generate platform-agnostic `poetry.lock` and `pdm.lock` files). As such,
Puffin's `requirements.txt` files may not be portable across platforms and Python versions.

## Advanced Usage

### Python discovery

Puffin itself does not depend on Python, but it does need to locate a Python environment to (1)
install dependencies into the environment, and (2) build source distributions.

When running `pip sync` or `pip install`, Puffin will search for a virtual environment in the
following order:

- An activated virtual environment based on the `VIRTUAL_ENV` environment variable.
- An activated Conda environment based on the `CONDA_PREFIX` environment variable.
- A virtual environment at `.venv` in the current directory, or in the nearest parent directory.

If no virtual environment is found, Puffin will prompt the user to create one in the current
directory via `puffin venv`.

When running `pip compile`, Puffin does not _require_ a virtual environment and will search for a
Python interpreter in the following order:

- An activated virtual environment based on the `VIRTUAL_ENV` environment variable.
- An activated Conda environment based on the `CONDA_PREFIX` environment variable.
- A virtual environment at `.venv` in the current directory, or in the nearest parent directory.
- The Python interpreter available as `python3` on the system path (preferring, e.g., `python3.7` if `--python-version=3.7` is specified).

### Dependency caching

Puffin uses aggressive caching to avoid re-downloading (and re-building dependencies) that have
already been accessed in prior runs.

The specifics of Puffin's caching semantics vary based on the nature of the dependency: 

- **For registry dependencies** (like those downloaded from PyPI), Puffin respects HTTP caching headers.
- **For direct URL dependencies**, Puffin respects HTTP caching headers, and also caches based on
  the URL itself.
- **For Git dependencies**, Puffin caches based on the fully-resolved Git commit hash. As such,
  `puffin pip compile` will pin Git dependencies to a specific commit hash when writing the resolved
  dependency set.
- **For local dependencies**, Puffin caches based on the last-modified time of the `setup.py` or
  `pyproject.toml` file.

If you're running into caching issues, Puffin includes a few escape hatches:

- To force Puffin to ignore cached data for all dependencies, run `puffin pip install --reinstall ...`.
- To force Puffin to ignore cached data for a specific dependency, run, e.g., `puffin pip install --reinstall-package flask ...`.
- To clear the global cache entirely, run `puffin clean`. 

### Resolution strategy

By default, Puffin follows the standard Python dependency resolution strategy of preferring the
latest compatible version of each package. For example, `puffin pip install flask>=2.0.0` will
install the latest version of Flask (at time of writing: `3.0.0`).

However, Puffin's resolution strategy be configured to prefer the _lowest_ compatible version of
each package (`--resolution=lowest`), or even the lowest compatible version of any _direct_
dependencies (`--resolution=lowest-direct`), both of which can be useful for library authors looking
to test their packages against the oldest supported versions of their dependencies.

For example, given the following `requirements.in` file:

```text
flask>=2.0.0
```

Running `puffin pip compile requirements.in` would produce the following `requirements.txt` file:

```text
# This file was autogenerated by Puffin v0.0.1 via the following command:
#    puffin pip compile requirements.in
blinker==1.7.0
    # via flask
click==8.1.7
    # via flask
flask==3.0.0
itsdangerous==2.1.2
    # via flask
jinja2==3.1.2
    # via flask
markupsafe==2.1.3
    # via
    #   jinja2
    #   werkzeug
werkzeug==3.0.1
    # via flask
```

However, `puffin pip compile --resolution=lowest requirements.in` would instead produce:

```text
# This file was autogenerated by Puffin v0.0.1 via the following command:
#    puffin pip compile requirements.in --resolution=lowest
click==7.1.2
    # via flask
flask==2.0.0
itsdangerous==2.0.0
    # via flask
jinja2==3.0.0
    # via flask
markupsafe==2.0.0
    # via jinja2
werkzeug==2.0.0
    # via flask
```

### Pre-release handling

By default, Puffin will accept pre-release versions during dependency resolution in two cases:

1. If the package is a direct dependency, and its version markers include a pre-release specifier
   (e.g., `flask>=2.0.0rc1`).
1. If _all_ published versions of a package are pre-releases.

If dependency resolution fails due to a transitive pre-release, Puffin will prompt the user to
re-run with `--prerelease=allow`, to allow pre-releases for all dependencies.

Alternatively, you can add the transitive dependency to your `requirements.in` file with 
pre-release specifier (e.g., `flask>=2.0.0rc1`) to opt in to pre-release support for that specific
dependency.

Pre-releases are [notoriously difficult](https://pubgrub-rs-guide.netlify.app/limitations/prerelease_versions)
to model, and are a frequent source of bugs in other packaging tools. Puffin's pre-release handling
is _intentionally_ limited and _intentionally_ requires user intervention to opt in to pre-releases
to ensure correctness, though pre-release handling will be revisited in future releases.

### Dependency overrides

Historically, `pip` has supported "constraints" (`-c constraints.txt`), which allows users to
narrow the set of acceptable versions for a given package.

Puffin supports constraints, but also takes this concept further by allowing users to _override_ the
acceptable versions of a package across the dependency tree via overrides (`-o overrides.txt`).

In short, overrides allow the user to lie to the resolver by overriding the declared dependencies
of a package. Overrides are a useful last resort for cases in which the user knows that a
dependency is compatible with a newer version of a package than the package declares, but the
package has not yet been updated to declare that compatibility.

For example, if a transitive dependency declares `pydantic>=1.0,<2.0`, but the user knows that
the package is compatible with `pydantic>=2.0`, the user can override the declared dependency
with `pydantic>=2.0,<3` to allow the resolver to continue.

While constraints are purely _additive_, and thus cannot _expand_ the set of acceptable versions for
a package, overrides _can_ expand the set of acceptable versions for a package, providing an escape
hatch for erroneous upper version bounds.

### Multi-version resolution

Puffin's `pip-compile` command produces a resolution that's known to be compatible with the
current platform and Python version. Unlike Poetry, PDM, and other package managers, Puffin does
not yet produce a machine-agnostic lockfile.

However, Puffin _does_ support resolving for alternate Python versions via the `--python-version`
command line argument. For example, if you're running Puffin on Python 3.9, but want to resolve for
Python 3.8, you can run `puffin pip compile --python-version=3.8 requirements.in` to produce a
Python 3.8-compatible resolution.

## Acknowledgements

Puffin's dependency resolver uses [PubGrub](https://github.com/pubgrub-rs/pubgrub) under the hood.
We're grateful to the PubGrub maintainers, especially [Jacob Finkelman](https://github.com/Eh2406),
for their support.

Puffin's Git implementation draws on details from [Cargo](https://github.com/rust-lang/cargo).

Some of Puffin's optimizations are inspired by the great work we've seen in
[Orogene](https://github.com/orogene/orogene) and [Bun](https://github.com/oven-sh/bun). We've also
learned a lot from [Posy](https://github.com/njsmith/posy).

## License

Puffin is licensed under either of

- Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or https://www.apache.org/licenses/LICENSE-2.0)
- MIT license ([LICENSE-MIT](LICENSE-MIT) or https://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in Puffin by you, as defined in the Apache-2.0 license, shall be
dually licensed as above, without any additional terms or conditions.

<div align="center">
  <a target="_blank" href="https://astral.sh" style="background:none">
    <img src="https://raw.githubusercontent.com/astral-sh/ruff/main/assets/svg/Astral.svg">
  </a>
</div>