pip/docs/html/reference/build-system/pyproject-toml.md

pyproject.toml

Modern Python packages can contain a pyproject.toml file, first introduced in {pep}518 and later expanded in {pep}517, {pep}621 and {pep}660. This file contains build system requirements and information, which are used by pip to build the package.

Build process

The overall process for building a package is:

• Create an isolated build environment.
• Populate the build environment with build dependencies.
• Generate the package's metadata, if necessary and possible.
• Generate a wheel for the package.

The wheel can then be used to perform an installation, if necessary.

Build Isolation

For building packages using this interface, pip uses an isolated environment. That is, pip will install build-time Python dependencies in a temporary directory which will be added to sys.path for the build commands. This ensures that build requirements are handled independently of the user's runtime environment.

For example, a project that needs an older version of setuptools to build can still be installed, even if the user has an newer version installed (and without silently replacing that version).

Build-time dependencies

Introduced in {pep}518, the build-system.requires key in the pyproject.toml file is a list of requirement specifiers for build-time dependencies of a package.

[build-system]
requires = ["setuptools ~= 58.0", "cython ~= 0.29.0"]

It is also possible for a build backend to provide dynamically calculated build dependencies, using {pep}517's get_requires_for_build_wheel hook. This hook will be called by pip, and dependencies it describes will also be installed in the build environment. For example, newer versions of setuptools expose the contents of setup_requires to pip via this hook.

Build-time requirement specifiers follow {pep}508, so it's possible to reference packages with URLs. For example:

[build-system]
requires = ["setuptools @ git+https://github.com/pypa/setuptools.git@main"]

Metadata Generation

Once the build environment has been created and populated with build-time dependencies, pip will usually need metadata about a package (name, version, dependencies, and more).

If {pep}517's prepare_metadata_for_build_wheel hook is provided by the build backend, that will be used to generate the packages' metadata. Otherwise, a wheel will be generated (as described below) and the metadata contained within such a wheel will be used.

Wheel Generation

For generating a wheel, pip uses the {pep}517 build_wheel hook that has to be provided by the build backend. The build backend will generate a wheel, which may involve compiling extension code written in C/C++ (or other languages).

Wheels generated using this mechanism can be cached for reuse, to speed up future installations.

Editable Installation

For performing editable installs, pip will use {pep}660 build_wheel_for_editable hook that has to be provided by the build backend. The wheels generated using this mechanism are not cached.

If this hook is missing on the build backend _and_ there's a `setup.py` file
in the project, pip will fallback to the legacy setup.py-based editable
installation.

This is considered a stopgap solution until setuptools adds support for
{pep}`660`, at which point this functionality will be removed; following pip's
regular {ref}`deprecation policy <Deprecation Policy>`.

Backend Configuration

Build backends have the ability to accept configuration settings, which can change the way the build is handled. These settings take the form of a series of key=value pairs. The user can supply configuration settings using the --config-settings command line option (which can be supplied multiple times, in order to specify multiple settings).

The supplied configuration settings are passed to every backend hook call.

Build output

It is the responsibility of the build backend to ensure that the output is in the correct encoding, as described in {pep}517. This likely involves dealing with the same challenges as pip has for legacy builds.

Fallback Behaviour

If a project does not have a pyproject.toml file containing a build-system section, it will be assumed to have the following backend settings:

[build-system]
requires = ["setuptools>=40.8.0", "wheel"]
build-backend = "setuptools.build_meta:__legacy__"

If a project has a build-system section but no build-backend, then:

• It is expected to include setuptools and wheel as build requirements. An error is reported if the available version of setuptools is not recent enough.

• The setuptools.build_meta:__legacy__ build backend will be used.

Disabling build isolation

This can be disabled using the --no-build-isolation flag -- users supplying this flag are responsible for ensuring the build environment is managed appropriately, including ensuring that all required build-time dependencies are installed, since pip does not manage build-time dependencies when this flag is passed.

Historical notes

As this feature was incrementally rolled out, there have been various notable changes and improvements in it.

• setuptools 40.8.0 is the first version of setuptools that offers a {pep}517 backend that closely mimics directly executing setup.py.
• Prior to pip 18.0, pip only supports installing build requirements from wheels, and does not support the use of environment markers and extras (only version specifiers are respected).
• Prior to pip 18.1, build dependencies using .pth files are not properly supported; as a result namespace packages do not work under Python 3.2 and earlier.