SE4Sci

Pip / uv and PyPI

• PyPI hosts packages for pip / uv
• pip: the canonical package manager for Python
• uv: a Rust rewrite of pip, 10-100x faster
  • 20% of PyPI downloads now come from uv!
  • Also has a high-level API

Conda/Mamba/MicroMamba/Pixi and conda-forge

• Conda: multipurpose packager with Python focus
  • Written in Python
  • Now contains the libmamba solver (in C++)
• mamba: a faster conda written in C++
• micromamba: Used to be different, now (2.0) a statically linked mamba
• pixi: a Rust rewrite of conda, with a new high-level API
  • No low-level API to match conda/mamba

Which to use?

• PyPI is the official ecosystem, most packages are there
• Package owners control PyPI, while conda-forge is community driven
• If you develop a package, PyPI is first
• Conda tools can use PyPI packages

So we'll focus on PyPI.

The course website is a pixi project, by the way.

Getting started with uv

There are lots of ways to install uv. It's just a single Rust compiled binary.

https://docs.astral.sh/uv/getting-started/installation/

curl -LsSf https://astral.sh/uv/install.sh | sh
wget -qO- https://astral.sh/uv/install.sh | sh
pipx install uv
pip install uv
cargo install --git https://github.com/astral-sh/uv uv
brew install uv
# Windows:
winget install --id=astral-sh.uv -e
powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

Using uv

Low level API:

uv pip <...>
uv venv <...>

High level API:

uv run <command...>

Using packages

You should never just run "pip install <package>"; installing into your global site-packages is a bad idea, and installing into your user site-packages is worse!

Python has a solution for this: virtual environments.

But if you just want to install a command-line tool, you don't have to set them up yourself!

uv tool install <package>
<package> <args...>

Quick run

You can even do it in one line:

uvx <package> <args...>

This will create a temporary virtual environment, install the package, run it. The environment is cached for a time, so reruns are fast.

(I believe this name was inspired by npx from JavaScript, which does the same thing.)

Quick scripts

You can get this functionality for scripts, as well:

# /// script
# dependencies = ["rich"]
# requires-python = ">=3.11"
# ///

import rich

rich.print("[blue]Hello, world!")

uv run myscripts.py <args...>

Virtual environments

uv venv

This creates a .venv folder with a virtual environment in it. You can then activate it:

source .venv/bin/activate  # Linux/macOS
.venv\Scripts\activate  # Windows

Note that uv commands will automatically use a .venv folder if it exists.

Using venvs

When a venv is "active", commands will run in that venv.

Most common command:

uv pip install -e. --group dev

You can always delete the venv and restart, rerunnning uv venv replaces the venv by default.

High level API

uv can help you setup and run a project. The high level API does the things you would normally do manually:

uv init <...> # start a new project
uv add <package> # add a package to the project
uv run <command...> # run a command in the project environment

For an existing project, you just need uv run!

Other uv features

See uv --help

• uv python can manage installed Pythons
• uv build can build packages
• uv publish can publish packages
• uv tree can show the dependency tree
• uv self can manage uv itself