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# 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:

`pyproject.toml` 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 `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

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

## Controlling `setup_requires`

Hint

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 This has been superseded by the `build-system.requires` key in `pyproject.toml` files (per PEP 518).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/

Historical context

`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.