Writing a Python package

Writing a Python package is fairly straightforward, especially for "Python-only" packages. In the second example we will build a package for numpy which contains compiled code.

A Python-only package

The following recipe uses the noarch: python setting to build a noarch package that can be installed on any platform without modification. This is very handy for packages that are pure Python and do not contain any compiled extensions.

Additionally, noarch: python packages work with a range of Python versions (contrary to packages with compiled extensions that are tied to a specific Python version).

recipe.yaml

context:
  version: "8.1.2"

package:
  name: ipywidgets
  version: ${{ version }}

source:
  url: https://pypi.io/packages/source/i/ipywidgets/ipywidgets-${{ version }}.tar.gz
  sha256: d0b9b41e49bae926a866e613a39b0f0097745d2b9f1f3dd406641b4a57ec42c9

build:
  noarch: python # (1)!
  script: pip install . -v

requirements:
  # note that there is no build section
  host:
    - pip
    - python >=3.7
    - setuptools
    - wheel
  run:
    - comm >=0.1.3
    - ipython >=6.1.0
    - jupyterlab_widgets >=3.0.10,<3.1.0
    - python >=3.7
    - traitlets >=4.3.1
    - widgetsnbextension >=4.0.10,<4.1.0

tests:
  - python:
      imports:
        - ipywidgets # (2)!

about:
  homepage: https://github.com/ipython/ipywidgets
  license: BSD-3-Clause
  license_file: LICENSE
  summary: Jupyter Interactive Widgets
  description: |
    ipywidgets are interactive HTML widgets for Jupyter notebooks and the IPython kernel.
  documentation: https://ipywidgets.readthedocs.io/en/latest/

1. The noarch: python line tells rattler-build that this package is pure Python and can be one-size-fits-all. noarch packages can be installed on any platform without modification which is very handy.
2. The imports section in the tests is used to check that the package is installed correctly and can be imported.

Running the recipe

To build this recipe, simply run:

rattler-build build --recipe ./ipywidgets

A Python package with compiled extensions

We will build a package for numpy – which contains compiled code. Since compiled code is python version-specific, we will need to specify the python version explicitly. The best way to do this is with a "variant_config.yaml" file:

variants.yaml

python:
  - 3.11
  - 3.12

This will replace any python found in the recipe with the versions specified in the variants.yaml file.

recipe.yaml

context:
  version: 2.0.1
  default_abi_level: 1.21

package:
  name: numpy
  version: ${{ version }}

source:
  - url: https://github.com/numpy/numpy/releases/download/v${{ version }}/numpy-${{ version }}.tar.gz
    sha256: 485b87235796410c3519a699cfe1faab097e509e90ebb05dcd098db2ae87e7b3

build:
  python:
    entry_points:
      - f2py = numpy.f2py.f2py2e:main  # [win]
      - numpy-config = numpy._configtool:main

requirements:
  build:
    - ${{ compiler('c') }}
    - ${{ compiler('cxx') }}
    # note: some `host` dependencies that run at build time (e.g., `cython`, `meson-python`)
    #       should ideally be in `build` instead, this is because cross compilation of
    #       Python packages in conda-forge uses `crossenv` rather than regular cross compilation.
  host:
    # note: variant is injected here!
    - python
    - pip
    - meson-python
    - pkg-config
    - python-build
    - cython
    - libblas
    - libcblas
    - liblapack
  run:
    - python
  run_exports:
    - numpy >=${{ default_abi_level }},<3.0.0a0

tests:
  - python:
      imports:
        - numpy
        - numpy.fft
        - numpy.linalg
        - numpy.random
        - numpy.ctypeslib

  - script:
    - f2py -v
    - numpy-config --cflags

about:
  homepage: http://numpy.org/
  license: BSD-3-Clause
  license_file: LICENSE.txt
  summary: The fundamental package for scientific computing with Python.
  documentation: https://numpy.org/doc/stable/
  repository: https://github.com/numpy/numpy

The build script for Unix:

build.sh

mkdir builddir

$PYTHON -m build -w -n -x \
    -Cbuilddir=builddir \
    -Csetup-args=-Dblas=blas \
    -Csetup-args=-Dlapack=lapack

$PYTHON -m pip install dist/numpy*.whl

The build script for Windows:

build.bat

mkdir builddir

%PYTHON% -m build -w -n -x ^
    -Cbuilddir=builddir ^
    -Csetup-args=-Dblas=blas ^
    -Csetup-args=-Dlapack=lapack
if %ERRORLEVEL% neq 0 exit 1

:: `pip install dist\numpy*.whl` does not work on windows,
:: so use a loop; there's only one wheel in dist/ anyway
for /f %%f in ('dir /b /S .\dist') do (
    pip install %%f
    if %ERRORLEVEL% neq 0 exit 1
)

Running the recipe

Running this recipe with the variant config file will build a total of 2 numpy packages:

rattler-build build --recipe ./numpy

At the beginning of the build process, rattler-build will print the following message to show you the variants it found:

Found variants:

numpy-1.26.4-py311h5f8ada8_0
╭─────────────────┬───────────╮
│ Variant         ┆ Version   │
╞═════════════════╪═══════════╡
│ python          ┆ 3.11      │
│ target_platform ┆ osx-arm64 │
╰─────────────────┴───────────╯

numpy-1.26.4-py312h440f24a_0
╭─────────────────┬───────────╮
│ Variant         ┆ Version   │
╞═════════════════╪═══════════╡
│ python          ┆ 3.12      │
│ target_platform ┆ osx-arm64 │
╰─────────────────┴───────────╯