Build Variants

Learn how to create build matrices that produce packages for different dependency versions, similar to parameterized builds or build configurations.

pixi-build is a preview feature and will change until stabilized. Keep this in mind when using it for your projects.

Why Use Build Variants?

Build variants solve version compatibility challenges:

Multiple Python versions - Support Python 3.11, 3.12, 3.13, etc.
Library versions - Build against different versions of dependencies
Testing - Verify compatibility across version ranges
Distribution - Provide packages for various configurations

In the conda ecosystem, this functionality is called “variants.” Other build systems might call this a build matrix, build configurations, or parameterized builds.

The Problem Variants Solve

Consider a C++ package built for Python 3.12:

[package.host-dependencies]
python = "3.12.*"

If a user tries to use this package with Python 3.11, Pixi reports a version conflict. Variants allow you to build multiple versions of the package, each compatible with different Python versions.

Creating Build Variants

Start with a Workspace

Begin with the workspace from the workspace tutorial:

workspace/
├── pixi.toml
├── pyproject.toml
├── src/
│   └── python_rich/
│       └── __init__.py
└── packages/
    └── cpp_math/
        ├── pixi.toml
        ├── CMakeLists.txt
        └── src/
            └── math.cpp

Relax Version Constraints

First, change the Python requirement in the C++ package to accept any version:

[package]
name = "cpp_math"
version = "0.1.0"

[package.host-dependencies]
cmake = ">=3.20, <3.27"
nanobind = ">=2.4.0, <2.5.0"
python = "*"  # (1)!

Changed from "3.12.*" to "*" to allow any Python version

Define Allowed Variants

Specify which Python versions to support:

[workspace]
channels = ["https://prefix.dev/conda-forge"]
platforms = ["osx-arm64", "osx-64", "linux-64", "win-64"]
preview = ["pixi-build"]

[workspace.build-variants]
python = ["3.11.*", "3.12.*"]  # (1)!

[dependencies]
python_rich = { path = "." }

Pixi will build packages for both Python 3.11 and 3.12

Without explicit environments, Pixi chooses a version based on the solver. To test both versions, create explicit environments.

Create Environments for Each Variant

Define environments that pin specific Python versions:

[feature.py311.dependencies]
python = "3.11.*"

[feature.py312.dependencies]
python = "3.12.*"

[environments]
py311 = ["py311"]  # (1)!
py312 = ["py312"]  # (2)!

Environment using Python 3.11

Environment using Python 3.12

Verify Different Builds

Check that different packages were built:

pixi list --environment py311

Package            Version     Build               Size       Kind   Source
python             3.11.11     h9e4cc4f_1_cpython  29.2 MiB   conda  python
cpp_math           0.1.0       py311h43a39b2_0                conda  cpp_math
python_rich        0.1.0       pyhbf21a9e_0                   conda  python_rich

pixi list --environment py312

Package            Version     Build               Size       Kind   Source
python             3.12.8      h9e4cc4f_1_cpython  30.1 MiB   conda  python
cpp_math           0.1.0       py312h2078e5b_0                conda  cpp_math
python_rich        0.1.0       pyhbf21a9e_0                   conda  python_rich

Notice the different build strings:

cpp_math has different builds: py311h43a39b2_0 vs py312h2078e5b_0
python_rich has the same build: pyhbf21a9e_0 (noarch Python package)

Complete Example

Here’s the full configuration:

pixi.toml

[workspace]
channels = ["https://prefix.dev/conda-forge"]
platforms = ["osx-arm64", "osx-64", "linux-64", "win-64"]
preview = ["pixi-build"]

[workspace.build-variants]
python = ["3.11.*", "3.12.*"]

[dependencies]
python_rich = { path = "." }

[feature.py311.dependencies]
python = "3.11.*"

[feature.py312.dependencies]
python = "3.12.*"

[environments]
py311 = ["py311"]
py312 = ["py312"]

[tasks]
start = "rich-example-main"

[package]
name = "python_rich"
version = "0.1.0"

[package.build]
backend = { name = "pixi-build-python", version = "0.4.*" }

[package.build.config]
noarch = false  # (1)!

[package.host-dependencies]
hatchling = "==1.26.3"
python = "*"  # (2)!

[package.run-dependencies]
cpp_math = { path = "packages/cpp_math" }
rich = ">=13.9.4,<14"

Disable noarch to create variant-specific builds
Allow any Python version - resolved by variants

Multiple Variant Dimensions

You can create variants for multiple dependencies:

[workspace.build-variants]
python = ["3.11.*", "3.12.*", "3.13.*"]
nanobind = ["2.3.*", "2.4.*"]

This creates a build matrix:

Python 3.11 + nanobind 2.3
Python 3.11 + nanobind 2.4
Python 3.12 + nanobind 2.3
Python 3.12 + nanobind 2.4
Python 3.13 + nanobind 2.3
Python 3.13 + nanobind 2.4

Be careful with variant matrices - they grow exponentially! 3 Python versions × 2 nanobind versions × 4 platforms = 24 packages.

Variant Files

For complex configurations, use external variant files:

pixi.toml

[workspace.build-variants]
files = ["variants.yaml"]

variants.yaml

python:
  - 3.11.*
  - 3.12.*
  - 3.13.*

nanobind:
  - 2.4.*
  - 2.5.*

zip_keys:
  - [python, nanobind]  # (1)!

Create paired variants instead of a full matrix

This creates only paired combinations:

Python 3.11 + nanobind 2.4
Python 3.12 + nanobind 2.5
Python 3.13 + nanobind 2.5

See the manifest reference for details.

Testing Across Variants

Run Tests in All Environments

pixi run --environment py311 test
pixi run --environment py312 test

Run Tasks in All Environments

Use a loop (bash):

for env in py311 py312 py313; do
  echo "Testing in $env"
  pixi run --environment $env test
done

Define Environment-Specific Tasks

[feature.py311.tasks]
test = "pytest tests/"

[feature.py312.tasks]
test = "pytest tests/ --strict-markers"

[feature.py313.tasks]
test = "pytest tests/ --strict-markers --new-feature"

Understanding Build Strings

Build strings indicate package variants:

cpp_math-0.1.0-py311h43a39b2_0.conda
                ^^^^^ ^^^^^^^^
                |     |
                |     Hash of dependencies
                Python version

Breakdown:

py311 - Python 3.11 variant
h43a39b2 - Hash of build configuration
_0 - Build number

Noarch packages (pure Python) have the same build string across variants:

python_rich-0.1.0-pyhbf21a9e_0.conda
                   ^^ ^^^^^^^^
                   |  |
                   |  Hash
                   Python (noarch)

Platform vs Variant Matrix

Platforms - Different OS/architectures:

[workspace]
platforms = ["linux-64", "osx-arm64", "win-64"]

Variants - Different dependency versions:

[workspace.build-variants]
python = ["3.11.*", "3.12.*"]

Combined: 3 platforms × 2 Python versions = 6 packages

Common Variant Patterns

[workspace.build-variants]
python = ["3.11.*", "3.12.*", "3.13.*"]

Advanced: Conditional Variants

Use variant files for platform-specific variants:

variants.yaml

python:
  - 3.11.*
  - 3.12.*

cuda:
  - 11.8  # linux-64 only
  - 12.0  # linux-64 only

pin_run_as_build:
  python:
    min_pin: x.x
    max_pin: x.x

Best Practices

Start with minimal variants

Begin with essential versions:

# Start simple
[workspace.build-variants]
python = ["3.11.*", "3.12.*"]

# Expand later
python = ["3.10.*", "3.11.*", "3.12.*", "3.13.*"]

Use version ranges in packages

Make packages flexible:

[package.host-dependencies]
python = "*"           # Any version
nanobind = ">=2.4,<3"  # Range

Test with CI/CD

Automate variant testing:

jobs:
  test:
    strategy:
      matrix:
        environment: [py311, py312, py313]
    steps:
      - run: pixi run --environment ${{ matrix.environment }} test

Document required variants

Explain why variants exist:

[workspace.build-variants]
# Support Python 3.11+ for modern type hints
# Support Python 3.12+ for better performance
python = ["3.11.*", "3.12.*"]

Next Steps

Dependency Types

Understand how variants affect dependencies

Workspaces

Use variants in multi-package workspaces

Build Backends

Backend-specific variant support

Manifest Reference

Complete variant configuration options

Troubleshooting

Variants not creating different builds

Check that:

Package has noarch = false (or omit it)
Dependency uses * or a range: python = "*"
Variant is actually used in dependencies

[package.build.config]
noarch = false  # Required for variants

[package.host-dependencies]
python = "*"  # Must allow variant resolution

Too many builds

Reduce variant dimensions:

# Instead of this (6 builds)
[workspace.build-variants]
python = ["3.11.*", "3.12.*", "3.13.*"]
nanobind = ["2.4.*", "2.5.*"]

# Do this (3 builds)
[workspace.build-variants]
python = ["3.11.*", "3.12.*", "3.13.*"]

Environment resolution fails

Ensure environment pinning matches variants:

[workspace.build-variants]
python = ["3.11.*", "3.12.*"]  # Must match environment pins

[feature.py311.dependencies]
python = "3.11.*"  # Matches variant

Get Started

Core Concepts

Tutorials

Building Packages

Distribution

Integration

Advanced

Build Variants

Why Use Build Variants?

The Problem Variants Solve

Creating Build Variants

Complete Example

Multiple Variant Dimensions

Variant Files

Testing Across Variants

Run Tests in All Environments

Run Tasks in All Environments

Define Environment-Specific Tasks

Understanding Build Strings

Platform vs Variant Matrix

Common Variant Patterns

Advanced: Conditional Variants

Best Practices

Next Steps

Dependency Types

Workspaces

Build Backends

Manifest Reference

Troubleshooting

Get Started

Core Concepts

Tutorials

Building Packages

Distribution

Integration

Advanced

​Why Use Build Variants?

​The Problem Variants Solve

​Creating Build Variants

​Complete Example

​Multiple Variant Dimensions

​Variant Files

​Testing Across Variants

​Run Tests in All Environments

​Run Tasks in All Environments

​Define Environment-Specific Tasks

​Understanding Build Strings

​Platform vs Variant Matrix

​Common Variant Patterns

​Advanced: Conditional Variants

​Best Practices

​Next Steps

Dependency Types

Workspaces

Build Backends

Manifest Reference

​Troubleshooting

Why Use Build Variants?

The Problem Variants Solve

Creating Build Variants

Complete Example

Multiple Variant Dimensions

Variant Files

Testing Across Variants

Run Tests in All Environments

Run Tasks in All Environments

Define Environment-Specific Tasks

Understanding Build Strings

Platform vs Variant Matrix

Common Variant Patterns

Advanced: Conditional Variants

Best Practices

Next Steps

Troubleshooting