Build System Support

What is it?

Python packaging has come a long way.

The traditional setuptools way of packaging Python modules uses a setup() function within the setup.py script. Commands such as python setup.py bdist or python setup.py bdist_wheel generate a distribution bundle and python setup.py install installs the distribution. This interface makes it difficult to choose other packaging tools without an overhaul. Because setup.py scripts allow for arbitrary execution, it is difficult to provide a reliable user experience across environments and history.

PEP 517 came to the rescue and specified a new standard for packaging and distributing Python modules. Under PEP 517:

A pyproject.toml file is used to specify which program to use to generate the distribution.

Two functions provided by the program, build_wheel(directory: str) and build_sdist(directory: str), create the distribution bundle in the specified directory.

The program may use its own configuration file or extend the .toml file.

The actual installation is done with pip install *.whl or pip install *.tar.gz. If *.whl is available, pip will go ahead and copy its files into the site-packages directory. If not, pip will look at pyproject.toml and decide which program to use to ‘build from source’. (Note that if there is no pyproject.toml file or the build-backend parameter is not defined, then the fall-back behaviour is to use setuptools.)

With this standard, switching between packaging tools is a lot easier.

How to use it?

Start with a package that you want to distribute. You will need your source files, a pyproject.toml file and a setup.cfg file:

~/meowpkg/
    pyproject.toml
    setup.cfg
    meowpkg/
                __init__.py
                    module.py

The pyproject.toml file specifies the build system (i.e. what is being used to package your scripts and install from source). To use it with setuptools the content would be:

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

build_meta implements setuptools’ build system support. The setuptools package implements the build_sdist command and the wheel package implements the build_wheel command; the latter is a dependency of the former exposed via PEP 517 hooks.

Use setuptools’ declarative config to specify the package information in setup.cfg:

[metadata]
name = meowpkg
version = 0.0.1
description = a package that meows

[options]
packages = find:

Now generate the distribution. To build the package, use PyPA build:

$ pip install -q build
$ python -m build

And now it’s done! The .whl file and .tar.gz can then be distributed and installed:

dist/
    meowpkg-0.0.1.whl
    meowpkg-0.0.1.tar.gz

$ pip install dist/meowpkg-0.0.1.whl

or:

$ pip install dist/meowpkg-0.0.1.tar.gz

Dynamic build dependencies and other build_meta tweaks

With the changes introduced by PEP 517 and PEP 518, the setup_requires configuration field was deprecated in setup.cfg and setup.py, in favour of directly listing build dependencies in the requires field of the build-system table of pyproject.toml. This approach has a series of advantages and gives package managers and installers the ability to inspect the build requirements in advance and perform a series of optimisations.

However, some package authors might still need to dynamically inspect the final user’s machine before deciding these requirements. One way of doing that, as specified by PEP 517, is to “tweak” setuptools.build_meta by using an in-tree backend.

Tip

Before implementing an in-tree backend, have a look at PEP 508. Most of the time, dependencies with environment markers are enough to differentiate operating systems and platforms.

If you put the following configuration in your pyproject.toml:

[build-system]
requires = ["setuptools"]
build-backend = "backend"
backend-path = ["_custom_build"]

then you can implement a thin wrapper around build_meta in the _custom_build/backend.py file, as shown in the following example:

from setuptools import build_meta as _orig

prepare_metadata_for_build_wheel = _orig.prepare_metadata_for_build_wheel
build_wheel = _orig.build_wheel
build_sdist = _orig.build_sdist


def get_requires_for_build_wheel(self, config_settings=None):
    return _orig.get_requires_for_build_wheel(config_settings) + [...]


def get_requires_for_build_sdist(self, config_settings=None):
    return _orig.get_requires_for_build_sdist(config_settings) + [...]

Note that you can override any of the functions specified in PEP 517, not only the ones responsible for gathering requirements.

Important

Make sure your backend script is included in the source distribution, otherwise the build will fail. This can be done by using a SCM/VCS plugin (like setuptools-scm and setuptools-svn), or by correctly setting up MANIFEST.in.

The generated .tar.gz and .whl files are compressed archives that can be inspected as follows: On POSIX systems, this can be done with tar -tf dist/*.tar.gz and unzip -l dist/*.whl. On Windows systems, you can rename the .whl to .zip to be able to inspect it from File Explorer. You can also use the above tar command in a command prompt to inspect the .tar.gz file. Alternatively, there are GUI programs like 7-zip that handle .tar.gz and .whl files.

In general, the backend script should be present in the .tar.gz (so the project can be built from the source) but not in the .whl (otherwise the backend script would end up being distributed alongside your package). See “Package Discovery and Namespace Packages” for more details about package files.