The story of distutils build directory in Gentoo
The Python distutils build system, as well as setuptools (that it was later merged into), used a two-stage build: first, a build command would prepare a built package version (usually just copy the .py files, sometimes compile Python extensions) into a build directory, then an install command would copy them to the live filesystem, or a staging directory. Curious enough, distutils were an early adopter of out-of-source builds — when used right (which often enough wasn’t the case), no writes would occur in the source directory and all modifications would be done directly in the build directory.
Today, in the PEP517 era, two-stage builds aren’t really relevant anymore. Build systems were turned into black boxes that spew wheels. However, setuptools still internally uses the two-stage build and the build directory, and therefore it still remains relevant to Gentoo eclasses. In this post, I’d like to shortly tell how we dealt with it over the years.