Using Pixi

name: using-pixi description: Guidance for developing under a projects managed by pixi. Always use when workspace managed by pixi (have pixi.toml or pixi.lock in root directory.) Covers all you should know while developing under a pixi-based project: pixi environment commands (install, run, shell, add, task), multi-environment management (-e flag, features), task configuration in pixi.toml, dependencies (conda vs PyPI), lock files, and development workflows. Helps with environment setup, package management, running tasks, debugging pixi.toml errors, and production deployment patterns.

Pixi Helper

Expert guidance for using Pixi, a fast conda-based package manager for Python projects.

Purpose

Help developers effectively use Pixi for managing conda/PyPI dependencies, running commands in isolated environments, and configuring multi-environment projects.

When to Use

• Working with pixi.toml or pixi.lock files
• Installing or managing packages
• Running commands or tasks in pixi environments
• Setting up development/production environments
• Debugging pixi configuration errors

Quick Command Reference

Essential Commands

# Initialize & install
pixi init [project-name]
pixi install                         # install default environment
pixi install -e dev                  # install dev environment

# Add dependencies
pixi add python=3.11 pytorch pandas  # conda packages
pixi add --pypi transformers         # PyPI packages
pixi add --feature dev pytest ruff   # to dev feature

# Run commands
pixi run python script.py            # use default env
pixi run -e dev pytest               # use dev env
pixi shell                           # interactive shell
pixi shell -e dev                    # dev shell

# Tasks
pixi task add lint "ruff check ."
pixi task list
pixi run lint

# Production
pixi run --frozen --no-install train.py

Key Flags

• -e <env>: Use specific environment
• --frozen: Don't update lock file (reproducible)
• --no-install: Skip installation
• --feature <name>: Target specific feature

Core Concepts

Workspace

Directory with:

• pixi.toml: Configuration (commit this)
• pixi.lock: Lock file (commit this)
• .pixi/: Environment cache (DON'T commit)

Environments

Named dependency sets. Use -e flag to select:

pixi run python script.py       # default
pixi run -e dev pytest           # dev
pixi run -e prod train.py        # prod

Features

Reusable dependency groups that compose into environments.

Tasks

Predefined commands in pixi.toml. Run with pixi run <task-name>.

Command Patterns

When to Use What

Adding Dependencies

Prefer conda first (better compatibility, faster):

pixi add numpy pytorch

Use PyPI when needed:

pixi add --pypi transformers

Why conda first? Pixi resolves conda deps first, then PyPI.

pixi.toml Configuration

Project Metadata

[project]
name = "my-project"
channels = ["conda-forge", "pytorch"]  # order matters
platforms = ["linux-64", "osx-arm64"]

Dependencies

[dependencies]
python = ">=3.10"
numpy = "*"
pytorch = ">=2.0,<3"

[pypi-dependencies]
transformers = "*"
my-pkg = { path = "./local", editable = true }

Tasks

Simple:

[tasks]
test = "pytest tests/"

With environment variables:

[tasks.train]
cmd = "python train.py"
env = { CUDA_VISIBLE_DEVICES = "0,1" }

With dependencies:

[tasks]
clean = "rm -rf __pycache__"
test = { cmd = "pytest", depends-on = ["clean"] }

Multi-line:

[tasks.submit]
cmd = """
python train.py \
  --config config.yaml \
  --output results/
"""

Features

[feature.dev.dependencies]
pytest = "*"
ruff = "*"

[feature.cuda.dependencies]
pytorch-cuda = "12.1"

[environments]
default = []
dev = ["dev"]
gpu = ["cuda"]
dev-gpu = ["dev", "cuda"]

Activation Environment

[activation.env]
CUDA_HOME = "/usr/local/cuda"
PROJECT_ROOT = "$PIXI_PROJECT_ROOT"

Common Workflows

Your Patterns (susser-tod)

Dev testing:

pixi run -e dev pytest ./tests

Type checking:

pixi run typecheck

Profiling:

pixi run -e dev nsys profile python train.py

Troubleshooting

1. Package Not Found

Checks:

1. Channel specified in channels = [...]?
2. Package name correct? Try: pixi search <pkg>
3. Try PyPI: pixi add --pypi <pkg>

2. Environment Not Activated

❌ Wrong: python script.py (bare python) ✅ Correct: pixi run python script.py

4. Lock File Conflicts

After merge conflicts:

pixi install              # regenerate lock
# or
rm pixi.lock && pixi install

5. Environment Variables Not Set

Variables from [activation.env] only work with:

• pixi run <cmd>
• Inside pixi shell

NOT with bare python outside pixi.

6. Dependency Version Conflicts

Use solve-group to isolate environments:

[environments]
default = { solve-group = "group1" }
dev = { features = ["dev"], solve-group = "group1" }  # shared
prod = { solve-group = "group2" }                     # isolated

Best Practices

Dependencies

✅ Conda first, PyPI when needed ✅ Pin critical versions: pytorch = "2.1.*" ✅ Version constraints: python = ">=3.10,<3.12" ❌ Don't mix conda + PyPI for same package

Lock Files

✅ Commit pixi.lock ✅ Use --frozen in CI/production

Environments

✅ Use features to organize deps ✅ Share solve-groups when possible ✅ 2-4 environments is ideal

Reference

• Detailed pixi.toml schema: See REFERENCE.md
• Official docs: https://pixi.sh/latest/
• pixi.toml spec: https://pixi.sh/latest/reference/pixi_toml/

Practical guidance for Pixi. For project-specific patterns, check your CLAUDE.md.