An extremely fast Python package and project manager, written in Rust.
fyn is an independent community fork of uv. It started on uv's foundation, but it now has its own commands, settings, defaults, and behavior, alongside reduced package-index request metadata, added features, and long-standing bug fixes. See MANIFESTO.md for the full story.
- 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-Agentheader to package indexes such as PyPI, instead ofuv/<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.
From PyPI:
# With pip.
pip install fyn# Or pipx.
pipx install fynOr build from source:
cargo install --path crates/fynSee the command line reference with fyn help.
The live docs site is duriantaco.github.io/fyn. The source of
truth still lives in docs/ in the repository.
Start here:
For CLI-specific help, use fyn help or fyn help <command>.
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 0.02msDefine tasks in your pyproject.toml and run them with fyn run:
[tool.fyn.tasks]
test = { cmd = "pytest -xvs", env = { PYTHONWARNINGS = "error" } }
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 check
# runs lint, then test
$ 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 -xvsTask env values are applied to the spawned command. For chained tasks, parent env values are
inherited by child tasks, and child task values take precedence. Extra arguments are supported for
cmd tasks, but not for chained tasks.
If fyn run cannot spawn a command, it tries to point you at the next step instead of stopping at a
raw OS error: fyn run --list-tasks for likely task typos, fyn tool run <command> for missing
Python-provided executables, or a path-specific hint for missing ./script-style commands.
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.
If you pass a path, fyn shell activates that environment directly. Otherwise it uses VIRTUAL_ENV
when set, then the discovered project environment, then a local .venv. Use --no-project to skip
project discovery and only check the current directory.
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.
fyn upgrade is the convenience form of running fyn lock --upgrade and then fyn sync.
Inspect the current project and environment state:
$ fyn status
current directory: /home/user/example
project directory: /home/user/example
managed project: yes
workspace root: /home/user/example
pyproject.toml: yes
fyn.lock: yes
pip-in-project: warn
environment: /home/user/example/.venv
python: /home/user/example/.venv/bin/python3 (3.12.0)Use --check to fail when obvious project checks do not pass, or --json for scripting and editor
integrations. In managed projects, --check also reports missing environments and .python-version
pins that do not satisfy requires-python, and prints hint: lines with the suggested fix.
$ fyn status --check
...
check: failed
issue: environment not found
hint: Run `fyn sync` or `fyn venv` to create the project environment.Inspect the current machine and environment before installing or reinstalling PyTorch:
$ fyn torch doctor
PyTorch doctor
recommended backend: cu130
next command:
fyn pip install torch torchvision torchaudio --torch-backend=cu130Use --json for scripting. fyn torch doctor reports the recommendation and current package state,
but does not modify pyproject.toml.
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]>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 0.5.0fyn 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-noneUse a specific Python version in the current directory:
$ fyn python pin 3.11
Pinned `.python-version` to `3.11`Use --python-downloads-json-url <source> when you need fyn python pin to resolve against a
custom Python downloads manifest instead of the default bundled metadata.
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 12msCreate a virtual environment:
$ fyn venv
Using Python 3.12.3
Creating virtual environment at: .venv
Activate with: source .venv/bin/activateInstall 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
...Keep your cache from growing unbounded:
export UV_CACHE_MAX_SIZE=2GOldest entries are automatically pruned after every command when the cache exceeds the limit.
Supports K, M, G, and T suffixes.
Use different lockfiles for different environments:
UV_LOCKFILE=linux.lock fyn lock
UV_LOCKFILE=macos.lock fyn lockEnvironment 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.
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 .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.
The same ones as uv: macOS, Linux, and Windows, across x86_64 and aarch64.
At the workflow level, often yes, but not as a drop-in replacement. Many commands and UV_*
environment variables carry over, but fyn now has fork-specific commands, config, defaults, and
behavior. Projects still need [tool.uv] renamed to [tool.fyn] and uv.lock renamed to
fyn.lock unless you override the lockfile name.
See MANIFESTO.md for the fuller comparison, or the table below for some of the larger user-visible differences:
| Area | uv | fyn |
|---|---|---|
| Config namespace and lockfile | [tool.uv], uv.lock |
[tool.fyn], fyn.lock |
| Package index User-Agent | uv/<version> plus LineHaul metadata |
Minimal fyn/<version> |
| Task runner | No [tool.uv.tasks] |
[tool.fyn.tasks] |
shell command |
No uv shell |
fyn shell |
upgrade command |
No uv upgrade |
fyn upgrade |
status command |
No uv status |
fyn status |
torch doctor command |
No uv torch doctor |
fyn torch doctor |
Managed-project pip policy |
No pip-in-project setting |
pip-in-project: warn, error, or allow |
| Cache size limit | No UV_CACHE_MAX_SIZE |
UV_CACHE_MAX_SIZE |
| Custom lockfile name | No UV_LOCKFILE |
UV_LOCKFILE |
fyn's dependency resolver uses PubGrub under the hood. We're grateful to the PubGrub maintainers, especially Jacob Finkelman, for their support.
fyn started as a fork of uv by Astral and still shares substantial ancestry with it.
Some of fyn's workflow UX, especially around task-running and future workflow ergonomics, has also been informed by Hatch.
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.
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.