pyproject.toml

New in version 10.0.

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.

Metadata Generation

New in version 19.0.

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

New in version 19.0.

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

New in version 21.3.

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.

Compatibility fallback

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 deprecation policy.

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.