pip/docs/html/reference/build-system/index.md

(build-interface)=

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:

:hidden:

pyproject-toml
setup-py

Backend API based

Standards-backed interface, that has explicit declaration and management of build dependencies.

setup.py based

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 backend API based build system interface, in the following scenarios:

• if a pyproject.toml file exists, or
• if {pypi}setuptools or {pypi}wheel is not installed.

Otherwise, the legacy build system interface is used. The intention is to switch to using the backend API 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. For the backend API based build system interface, typically this will take the form of a message like¹:

Building wheel for pip (<build backend identification>)... done

For the setup.py based build system interface, typically this will look like the following:

Building wheel for pip (setup.py)... done

The output uses "pyproject.toml" instead of "PEP 517" to refer to be
*backend API* based build system interface.

The output uses the backend specification instead of "pyproject.toml"
to refer to be *backend API* based build system interface.

Controlling which build system interface is used

The --use-pep517 flag (and corresponding environment variable: PIP_USE_PEP517) can be used to force all packages to build using the backend API based build system interface. There is no way to force the use of the legacy build system interface.

(controlling-setup_requires)=

Controlling setup_requires

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.

The 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 approach yet!).

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 setup_requires 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

  [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 `setup_requires`
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.

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1. Here <build backend identification> is replaced with the actual value for the given package, e.g. Building wheel for pip (setuptools.build_meta)... done ↩︎