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# Python package guidelines

From ArchWiki

This document covers standards and guidelines on writing PKGBUILDs for Python software.

## Package naming

For Python 3 library modules, use `python-modulename`. Also use the prefix if the package provides a program that is strongly coupled to the Python ecosystem (e.g. pip or tox). For other applications, use only the program name.

Note The package name should be entirely lowercase.

## Architecture

See PKGBUILD#arch.

A Python package that contains C extensions is architecture-dependent. Otherwise it is most likely architecture-independent.

Packages built using setuptools define their C extensions using the `ext_modules` keyword in `setup.py`.

## Source

Note With RFC0020 the default is to use upstream provided source tarballs, instead of PyPI provided sdist tarballs.

Download URLs linked from the PyPI website include an unpredictable hash that needs to be fetched from the PyPI website each time a package must be updated. This makes them unsuitable for use in a PKGBUILD. PyPI provides an alternative stable scheme: PKGBUILD#source `source=()` array should use the following URL templates:

Source package
  `https://files.pythonhosted.org/packages/source/${_name::1}/${_name//-/_}/${_name//-/_}-$pkgver.tar.gz`
Pure Python wheel package
  `https://files.pythonhosted.org/packages/py2.py3/${_name::1}/$_name/${_name//-/_}-$pkgver-py2.py3-none-any.whl` (Bilingual – Python 2 and Python 3 compatible)
  `https://files.pythonhosted.org/packages/py3/${_name::1}/$_name/${_name//-/_}-$pkgver-py3-none-any.whl` (Python 3 only)
  Note that the distribution name can contain dashes, while its representation in a wheel filename cannot (they are converted to underscores).
Architecture specific wheel package
  Additional architecture-specific arrays can be added by appending an underscore and the architecture name, e.g. `source_x86_64=('...')`. Also `_py=cp310` can be used to not repeat the Python version:
  `https://files.pythonhosted.org/packages/$_py/${_name::1}/$_name/${_name//-/_}-$pkgver-$_py-${_py}m-manylinux1_x86_64.whl`

Note that a custom `_name` variable is used instead of `pkgname` since Python packages are generally prefixed with `python-`. This variable can generically be defined as follows:

    _name=${pkgname#python-}

## Installation methods

Python packages are generally installed using language-specific package manager such as pip, which fetches packages from an online repository (usually PyPI, the Python Package Index) and tracks the relevant files.

However, for managing Python packages from within `PKGBUILD`s, one needs to "install" the Python package to the temporary location `$pkgdir/usr/lib/python<Python version>/site-packages/$pkgname`.

For Python packages using standard metadata to specify their build backend in `pyproject.toml`, this can most easily achieved using python-build and python-installer. Old packages might fail to specify that they use setuptools, and only offer a `setup.py` that has to be invoked manually.

Note Dependencies from the package's metadata must be defined in the `depends` array otherwise they will not be installed.

### Standards based (PEP 517)

Tip When building from upstream provided source tarballs and upstream relies on git to derive a version string for the project, it is required to set tooling specific environment variables to `$pkgver` before building a wheel:

- python-flit-core, python-hatch-vcs and python-setuptools-scm: `SETUPTOOLS_SCM_PRETEND_VERSION`
- python-pbr: `PBR_VERSION`
- python-pdm-backend: `PDM_BUILD_SCM_VERSION`

A standards based workflow is straightforward: Build a wheel using python-build and install it to `$pkgdir` using python-installer:

Note Besides python-build and python-installer, you will also need to add the build backend used by the package to `makedepends`. All build backends that are available in the repo are members of the python-build-backend group. Check the `build-system.build-backend` in the project's `pyproject.toml` which is applicable for your project. If it is not configured, it defaults to python-setuptools.

    makedepends=(python-build python-installer python-wheel)

    build() {
        cd $_name-$pkgver
        python -m build --wheel --no-isolation
    }

    package() {
        cd $_name-$pkgver
        python -m installer --destdir="$pkgdir" dist/*.whl
    }

where:

- `--wheel` results in only a wheel file to be built, no source distribution.
- `--no-isolation` means that the package is built using what is installed on your system (which includes packages you specified in `depends`), by default the tool creates an isolated virtual environment and performs the build there.
- `--destdir="$pkgdir"` prevents trying to directly install in the host system instead of inside the package file, which would result in a permission error
- `--compile-bytecode=...` or `--no-compile-bytecode` can be passed to `installer`, but the default is sensibly picked, so this should not be necessary.

Note Skipping `build` and putting the `.whl` file in your `source` array is discouraged in favor of building from source, and should only be used when the latter is not a viable option (for example, packages which only come with wheel sources, and therefore cannot be built from source).
Tip If your package is a VCS package (`python-…-git`), include the command `git -C "${srcdir}/${pkgname}" clean -dfx` in your `prepare` function. This removes stale wheels along with other build artifacts, and helps prevent issues further down the road. See also upstream issues for setuptools and Poetry.

### setuptools or distutils

If no `pyproject.toml` can be found or it fails to contain a `[build-system]` table, it means the project is using the old legacy format, where the project provides a setup.py file, which invokes the `setup` function from setuptools or distutils.core.

Such packages usually can also be built and installed using the method described above using python-build and python-installer, and this is the preferred method. But they will also additionally need python-setuptools in `makedepends`.

You can still build and install these packages using the old, deprecated way of running setup.py directly, which is shown below and it is described here for those cases, where the pep-517 compliant way does not work for some reason.

But note that you will get the following warning in the output of the `package` step, when building a package using this method:

    SetuptoolsDeprecationWarning: setup.py install is deprecated.

Also note that Python versions from 3.12 onwards do not include distutils in the standard library any more, which means that packages for projects still using setup.py generally need to have python-setuptools in `makedepends`, since that provides its own version of distutils.

    makedepends=('python-setuptools')

    build() {
        cd $_name-$pkgver
        python setup.py build
    }

    package() {
        cd $_name-$pkgver
        python setup.py install --root="$pkgdir" --optimize=1
    }

where:

- `--root="$pkgdir"` works like `--destdir` above
- `--optimize=1` compiles optimized bytecode files (.opt-1.pyc) so they can be tracked by pacman instead of being created on the host system on demand.
- Adding `--skip-build` optimizes away the unnecessary attempt to re-run the build steps already run in the `build()` function, if that is the case.

If a package uses python-setuptools-scm, the package most likely will not build with an error such as:

    LookupError: setuptools-scm was unable to detect version for /build/python-jsonschema/src/jsonschema-3.2.0.

    Make sure you're either building from a fully intact git repository or PyPI tarballs. Most other sources (such as GitHub's tarballs, a git checkout without the .git folder) don't contain the necessary metadata and will not work.

To get it building `SETUPTOOLS_SCM_PRETEND_VERSION` has to be exported as an environment variable with `$pkgver` as the value:

    export SETUPTOOLS_SCM_PRETEND_VERSION=$pkgver

## Check

Note

- Avoid using `tox` to run testsuites as it is explicitly designed to test repeatable configurations downloaded from PyPI while `tox` is running, and does not test the version that will be installed by the package. This defeats the purpose of having a check function at all.
- Avoid adding pytest plugins for linting, coverage or type checking to checkdepends (see the #Disable pytest options section for details). This makes bootstrapping harder and these tests are not required for distribution packaging as they do not test functionality.
- Avoid using relative paths when setting a custom `PYTHONPATH` for tests, as it may lead to non-obvious error cases. Use absolute paths instead.

Most Python projects providing a testsuite use the unittest runner or nosetests or pytest (provided by python and python-nose and python-pytest, respectively) to run tests with `test` in the name of the file or directory containing the testsuite. In general, simply running `nosetests` or `pytest` is enough to run the testsuite.

    check(){
        cd $_name-$pkgver

        # Builtin unittest
        python -m unittest discover -v

        # For nosetests
        nosetests

        # For pytest
        pytest
    }

If there is a compiled C extension, the tests need to be run using a `$PYTHONPATH`, that reflects the current major and minor version of Python in order to find and load it.

    check(){
        cd $_name-$pkgver
        local python_version=$(python -c 'import sys; print("".join(map(str, sys.version_info[:2])))')

        # Builtin unittest
        PYTHONPATH="$PWD/build/lib.linux-$CARCH-cpython-$python_version" python -m unittest discover -vs .

        # For nosetests
        PYTHONPATH="$PWD/build/lib.linux-$CARCH-cpython-$python_version" nosetests

        # For pytest
        PYTHONPATH="$PWD/build/lib.linux-$CARCH-cpython-$python_version" pytest
    }

## Tips and tricks

### Using Python version

Sometimes during preparing, building, testing or installation it is required to refer to the system's major and minor Python version (e.g. `3.9` or `3.10`). Do not hardcode this and instead use a call to the Python interpreter to retrieve the information and store it in a local variable:

    check(){
      local python_version=$(python -c 'import sys; print(".".join(map(str, sys.version_info[:2])))')
      ...
    }

### Using site-packages

Sometimes during building, testing or installation it is required to refer to the system's `site-packages` directory. Do not hardcode this directory and use a call to the Python interpreter instead to retrieve the path and store it in a local variable:

    check(){
      local site_packages=$(python -c "import site; print(site.getsitepackages()[0])")
      ...
    }

### Test directory in site-package

Make sure to not install a directory named just `tests/` directly under `site-packages/` (i.e. `/usr/lib/pythonX.Y/site-packages/tests/`). Doing so could lead to conflicts between packages. Python package projects using setuptools are sometimes misconfigured to include the directory containing its tests as a top level Python package. If you encounter this, you can help by filing an issue with the package project and ask them to fix this, e.g. like this.

### Disable pytest options

When running pytest, it is mostly desirable to not run with additional plugins. Especially plugins for linting and coverage are counterproductive in packaging, as changes in behavior may have tests fail. To disable pytest options such as `addopts` it is preferred to use an option override on the command line over patching any configuration files used by pytest due to the maintenance overhead.

To unset all additional options use

    pytest -o addopts=""

### Fix reproducibility issue with meson-python

When using meson-python as a PEP 517 build backend it uses randomized folder paths that create reproducibility issues. This can be circumvented by hardcoding the used folder with the `-Cbuild-dir` flag:

    python -m build --wheel --no-isolation -Cbuild-dir=build

### Running tests with an installed package

Some packages require being installed to have the tests run successfully. In such cases, such as for example python-narwhals, you can create a virtual environment to install the built package and run the tests there.

    python -m venv --system-site-packages test-env
    test-env/bin/python -m installer dist/*.whl
    test-env/bin/python -P -m pytest

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