Build System Interface¶
When dealing with installable source distributions of a package, pip does not directly handle the build process for the package. This responsibility is delegated to “build backends” -- also known as “build systems”. This means that pip needs an interface, to interact with these build backends.
There are two main interfaces that pip uses for these interactions:
Standards-backed interface, that has explicit declaration and management of build dependencies.
Legacy interface, that we’re working to migrate users away from. Has no good mechanisms to declare build dependencies.
Details on the individual interfaces can be found on their dedicated pages, linked above. This document covers the nuances around which build system interface pip will use for a project, as well as details that apply to all the build system interfaces that pip may use.
Determining which build system interface is used¶
Currently, pip uses the
pyproject.toml based build system interface, if a
pyproject.toml file exists. If not, the legacy build system interface is used.
The intention is to switch to using the
pyproject.toml build system interface
unconditionally and to drop support for the legacy build system interface at
some point in the future.
When performing a build, pip will mention which build system interface it is using. Typically, this will take the form of a message like:
Building wheel for pip (pyproject.toml)... done
Building wheel for pip (setup.py)... done
The content in the brackets, refers to which build system interface is being used.
Changed in version 21.3: The output uses “pyproject.toml” instead of “PEP 517” to refer to be
pyproject.toml based build system interface.
Controlling which build system interface is used¶
--use-pep517 flag (and corresponding environment
PIP_USE_PEP517) can be used to force all packages to build using
pyproject.toml based build system interface. There is no way to force
the use of the legacy build system interface.
This is only relevant for projects that use setuptools as the build backend,
and use the
setup_requires keyword argument in their setup.py file.
setup_requires argument in
setup.py is used to specify build-time
dependencies for a package. This has been superseded by the
build-system.requires key in
pyproject.toml files (per PEP 518).
However, there are situations where you might encounter a package that uses
setup_requires (eg: the package has not been updated to use the newer
If you control the package, consider adding a
pyproject.toml file to utilise
the modern build system interface. That avoids invoking the problematic
behaviour by deferring to pip for the installations.
For the end users, the best solution for dealing with packages with
setup_requires is to install the packages listed in
beforehand, using a prior
pip install command. This is because there is no
way to control how these dependencies are located by
easy_install, or how
setuptools will invoke
pip using pip’s command line options -- which makes it
tricky to get things working appropriately.
If you wish to ensure that
easy_install invocations do not reach out to PyPI,
you will need to configure its behaviour using a
distutils configuration file. Here are some examples:
To have the dependency located at an alternate index with
[easy_install] index_url = https://my.index-mirror.com
To have the dependency located from a local directory and not crawl PyPI, add this:
[easy_install] allow_hosts = '' find_links = file:///path/to/local/archives/
setuptools < 52.0 will use
easy_install to try to fulfill
dependencies, which can result in weird failures --
easy_install does not
understand many of the modern Python packaging standards, and will usually
attempt to install incompatible package versions or to build packages
incorrectly. It also generates improper script wrappers, which don’t do the
right thing in many situations.
Newer versions of
setuptools will use
pip for these installations, but have
limited ability to pass through any command line arguments. This can also result
in weird failures and subtly-incorrect behaviour.