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.
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.
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
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).
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
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"]
New in version 19.0.
Once the build environment has been created and populated with build-time
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.
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
Wheels generated using this mechanism can be cached for reuse, to speed up future installations.
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.
If this hook is missing on the build backend and there’s a
in the project, pip will fallback to the legacy setup.py-based editable
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 backends have the ability to accept configuration settings, which can
change the way the build is handled. These settings take the form of a
key=value pairs. The user can supply configuration settings
--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.
Configuration settings provided via
--config-settings command line options (or the
equivalent environment variables or configuration file entries) are passed to the build
of requirements explicitly provided as pip command line arguments. They are not passed
to the build of dependencies, or to the build of requirements provided in requirement
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.
If a project does not have a
pyproject.toml file containing a
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
It is expected to include
wheelas build requirements. An error is reported if the available version of
setuptoolsis not recent enough.
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
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
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
.pthfiles are not properly supported; as a result namespace packages do not work under Python 3.2 and earlier.