=======================================
Build System Support
=======================================
What is it?
-------------
Python packaging has come `a long way <https://bernat.tech/posts/pep-517-518/>`_.
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``' :ref:`declarative config <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:
.. _building:
Now generate the distribution. To build the package, use
`PyPA build <https://pypa-build.readthedocs.io/en/latest/>`_::
$ 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
.. _backend-wrapper:
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
:pep:`in-tree backend <517#in-tree-build-backends>`.
.. tip:: Before implementing an *in-tree* backend, have a look at
:pep:`PEP 508 <508#environment-markers>`. 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``:
.. code-block:: 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:
.. code-block:: python
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(config_settings=None):
return _orig.get_requires_for_build_wheel(config_settings) + [...]
def get_requires_for_build_sdist(config_settings=None):
return _orig.get_requires_for_build_sdist(config_settings) + [...]
Note that you can override any of the functions specified in :pep:`PEP 517
<517#build-backend-interface>`, not only the ones responsible for gathering
requirements.
.. important:: Make sure your backend script is included in the :doc:`source
distribution </userguide/distribution>`, otherwise the build will fail.
This can be done by using a SCM_/VCS_ plugin (like :pypi:`setuptools-scm`
and :pypi:`setuptools-svn`), or by correctly setting up :ref:`MANIFEST.in
<manifest>`.
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 ":doc:`/userguide/package_discovery`" for more details about package
files.
.. _SCM: https://en.wikipedia.org/wiki/Software_configuration_management
.. _VCS: https://en.wikipedia.org/wiki/Version_control
.. _7-zip: https://www.7-zip.org
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