Python package guidelines

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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.

The same applies to Python 2 except that the prefix (if needed) is python2-.

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

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
in this example for 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 respository (usually PyPI, the Python Package Index) and tracks the relevant files.

However, for managing Python packages from within PKGBUILDs, 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)

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

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

build() {
    python -m build --wheel --no-isolation
}

package() {
    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.
Warning: 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).

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, which uses a setup.py file which invokes setuptools or distutils. Note that while distutils is included in Python's standardlib, having setuptools installed means that you use a patched version of distutils. 

makedepends=('python-setuptools')  # unless it only requires distutils

build() {
    python setup.py build
}

package() {
    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 the resulting package includes executables which import the deprecated pkg_resources module, then setuptools must be additionally specified as a depends in the split package_*() functions; alternatively, if the PKGBUILD only installs the Python package for a single version of Python, setuptools should be moved from makedepends to depends.

Some packages try to use setuptools and fall back to distutils if setuptools could not be imported. In this case, setuptools should be added as a makedepends, so that the resulting python metadata is better.

If a package needs setuptools to build due to including executables (which is not supported by distutils), but only imports distutils, then building will raise a warning, and the resulting package will be broken (it will not contain the executables): 

/usr/lib/python3.8/distutils/dist.py:274: UserWarning: Unknown distribution option: 'entry_points'
  warnings.warn(msg)

An upstream bug should be reported. To work around the problem, an undocumented setuptools feature can be used: 

# fails because of distutils
python setup.py build

# works by using a setuptools shim
python -m setuptools.launch setup.py build

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

Warning: 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.

Most python projects providing a testsuite use nosetests or pytest 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 "$srcdir/foo-$pkgver"

    # 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 "$pkgname-$pkgver"
  local python_version=$(python -c 'import sys; print(".".join(map(str, sys.version_info[:2])))')
  # For nosetests
  PYTHONPATH="$PWD/build/lib.linux-$CARCH-${python_version}" nosetests

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

Some projects provide setup.py entry points for running the test. This works for both pytest and nosetests. 

check(){
    cd "$srcdir/foo-$pkgver"

    # For nosetests
    python setup.py nosetests

    # For pytest - needs python-pytest-runner
    python setup.py pytest
}

Tips and tricks

Discovering detached PGP signatures on PyPI

If detached PGP signatures for a given Python sdist tarball exist, they should be used to verify the tarball. However, the signature files do not show up directly in the files download section of any given project on pypi.org. To discover the sdist tarballs and their potential signature files, it is possible to use this service to get an overview per project: https://pypi.debian.net/

For python-requests, this would be https://pypi.debian.net/requests.

Using site-packages

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

check(){
  cd "$pkgname-$pkgver"
  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 into site-packages (e.g. /usr/lib/python2.7/site-packages/tests/), as it easily conflicts with other Python packages.