fyn 0.10.14

An extremely fast Python package and project manager, written in Rust.

fyn

An extremely fast Python package and project manager, written in Rust.

fyn is an independent Python package manager built on uv's foundation, with reduced package-index request metadata, new features added, and long-standing bugs fixed. See MANIFESTO.md for the full story.

Highlights

• A single tool to replace pip, pip-tools, pipx, poetry, pyenv, twine, virtualenv, and more.
• 10-100x faster than pip.
• Provides comprehensive project management, with a universal lockfile.
• Built-in task runner — define and run project tasks in pyproject.toml.
• Activates virtual environments with fyn shell.
• Upgrades dependencies in one command with fyn upgrade.
• Runs scripts, with support for inline dependency metadata.
• Installs and manages Python versions.
• Runs and installs tools published as Python packages.
• Includes a pip-compatible interface for a performance boost with a familiar CLI.
• Supports Cargo-style workspaces for scalable projects.
• Disk-space efficient, with a global cache for dependency deduplication.
• Reduced package-index request metadata — compared with upstream uv, fyn sends a minimal fyn/<version> User-Agent header to package indexes such as PyPI, instead of uv/<version> plus the extra LineHaul environment metadata uv included. This reduces what is exposed in that header, but package indexes still receive normal network and request information.
• Supports macOS, Linux, and Windows.

Installation

From PyPI:

# With pip.
pip install fyn

# Or pipx.
pipx install fyn

Or build from source:

cargo install --path crates/fyn

See the command line reference documentation with fyn help.

Features

Projects

fyn manages project dependencies and environments, with support for lockfiles, workspaces, and more, similar to rye or poetry:

$ fyn init example
Initialized project `example` at `/home/user/example`

$ cd example

$ fyn add ruff
Creating virtual environment at: .venv
Resolved 2 packages in 170ms
Installed 2 packages in 1ms
 + ruff==0.5.0

$ fyn run ruff check
All checks passed!

$ fyn lock
Resolved 2 packages in 0.33ms

$ fyn sync
Resolved 2 packages in 0.70ms
Checked 1 package in 0.02ms

Task runner

Define tasks in your pyproject.toml and run them with fyn run:

[tool.fyn.tasks]
test = "pytest -xvs"
lint = "ruff check ."
format = { cmd = "ruff format .", description = "Format code" }
check = { chain = ["lint", "test"], description = "Lint then test" }

$ fyn run test
# runs pytest -xvs

$ fyn run test -- -k mytest
# extra args are passed through

$ fyn run --list-tasks
Available tasks:
  check    Lint then test
  format   Format code
  lint     ruff check .
  test     pytest -xvs

Shell activation

Activate the project's virtual environment in a new shell:

$ fyn shell
success: Activated virtual environment at .venv
Type exit to deactivate.

Works with bash, zsh, fish, nushell, powershell, and cmd.

Upgrade dependencies

Upgrade all or specific dependencies in one command:

$ fyn upgrade
info: Upgrading all dependencies...
success: Dependencies upgraded successfully.

$ fyn upgrade requests flask
info: Upgrading: requests, flask
success: Dependencies upgraded successfully.

Supports --dry-run and --no-sync.

Scripts

fyn manages dependencies and environments for single-file scripts.

Create a new script and add inline metadata declaring its dependencies:

$ echo 'import requests; print(requests.get("https://example.com"))' > example.py

$ fyn add --script example.py requests
Updated `example.py`

Then, run the script in an isolated virtual environment:

$ fyn run example.py
Reading inline script metadata from: example.py
Installed 5 packages in 12ms
<Response [200]>

Tools

fyn executes and installs command-line tools provided by Python packages, similar to pipx.

Run a tool in an ephemeral environment using fynx (an alias for fyn tool run):

$ fynx pycowsay 'hello world!'
Resolved 1 package in 167ms
Installed 1 package in 9ms
  """

  ------------
< hello world! >
  ------------
   \   ^__^
    \  (oo)\_______
       (__)\       )\/\
           ||----w |
           ||     ||

Install a tool with fyn tool install:

$ fyn tool install ruff
Resolved 1 package in 6ms
Installed 1 package in 2ms
 + ruff==0.5.0
Installed 1 executable: ruff

$ ruff --version
ruff 0.5.0

Python versions

fyn installs Python and allows quickly switching between versions.

Install multiple Python versions:

$ fyn python install 3.12 3.13 3.14
Installed 3 versions in 972ms
 + cpython-3.12.12-macos-aarch64-none
 + cpython-3.13.9-macos-aarch64-none
 + cpython-3.14.0-macos-aarch64-none

Use a specific Python version in the current directory:

$ fyn python pin 3.11
Pinned `.python-version` to `3.11`

The pip interface

fyn provides a fast, pip-compatible interface for common pip, pip-tools, and virtualenv workflows.

For many common workflows, you can switch to the fyn pip interface with minimal changes and keep the same overall workflow shape, while getting a 10-100x speedup.

Compile requirements into a platform-independent requirements file:

$ fyn pip compile requirements.in \
   --universal \
   --output-file requirements.txt
Resolved 43 packages in 12ms

Create a virtual environment:

$ fyn venv
Using Python 3.12.3
Creating virtual environment at: .venv
Activate with: source .venv/bin/activate

Install the locked requirements:

$ fyn pip sync requirements.txt
Resolved 43 packages in 11ms
Installed 43 packages in 208ms
 + babel==2.15.0
 + certifi==2024.7.4
 ...

Cache size limit

Keep your cache from growing unbounded:

export UV_CACHE_MAX_SIZE=2G

Oldest entries are automatically pruned after every command when the cache exceeds the limit. Supports K, M, G, and T suffixes.

Custom lockfile name

Use different lockfiles for different environments:

UV_LOCKFILE=linux.lock fyn lock
UV_LOCKFILE=macos.lock fyn lock

Private index support

Environment variables work in index URLs — useful for private indexes with credentials:

[[tool.fyn.index]]
name = "private"
url = "https://${PYPI_TOKEN}@pypi.example.com/simple/"

Explicit indexes are also respected for transitive dependencies, so you don't have to list every internal package as a direct dependency.

Migrating from uv

fyn is close to uv, but not a zero-edit rename. Most command-line workflows and UV_* environment variables carry over, but project metadata and lockfile names differ.

# 1. Rename your lockfile
mv uv.lock fyn.lock

# 2. In pyproject.toml, rename [tool.uv] to [tool.fyn]
sed -i 's/\[tool\.uv\]/[tool.fyn]/' pyproject.toml

# 3. Use fyn instead of uv
fyn sync
fyn run pytest
fynx ruff check .

Contributing

We are passionate about supporting contributors of all levels of experience and would love to see you get involved in the project. See the contributing guide to get started.

FAQ

What platforms does fyn support?

The same ones as uv: macOS, Linux, and Windows, across x86_64 and aarch64.

Is fyn compatible with uv?

Mostly at the workflow level, but not as a byte-for-byte drop-in replacement. Commands and many UV_* environment variables carry over, but projects need [tool.uv] renamed to [tool.fyn] and uv.lock renamed to fyn.lock unless you override the lockfile name.

What's different from uv?

See MANIFESTO.md for the full comparison, or the table below for a quick summary:

Feature	uv	fyn
Package index User-Agent	uv/<version> plus LineHaul metadata	Minimal fyn/<version>
Task runner	Not available	[tool.fyn.tasks]
shell command	Not available	fyn shell
upgrade command	Must chain two commands	fyn upgrade
Cache size limit	No limit	UV_CACHE_MAX_SIZE
Custom lockfile name	Not available	UV_LOCKFILE

Acknowledgements

fyn's dependency resolver uses PubGrub under the hood. We're grateful to the PubGrub maintainers, especially Jacob Finkelman, for their support.

fyn's core is derived from uv by Astral.

fyn's Git implementation is based on Cargo.

Some of fyn's optimizations are inspired by the great work we've seen in pnpm, Orogene, and Bun. We've also learned a lot from Nathaniel J. Smith's Posy and adapted its trampoline for Windows support.

License

fyn is licensed under either of

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

at your option.

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