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======================================= 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 <https://www.python.org/dev/peps/pep-0517/>`_ 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(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:`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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