Comment by HiPhish

> Python setup.py -> pyproject.toml?

That's because setup.py was objectively a mistake. In order to know how to run the package you need to know its dependencies, which requires evaluating setup.py, but you cannot evaluate setup.py until you know the dependencies, which requires you to evaluate setup.py, and so on. The pyproject.toml is plain text where you specify the build backend and its options, and the build system can be as complex of a program as need by.

If anything, pyproject.toml is an example of taking the idea of programmable configuration to its next conclusion. The build system is part of the configuration and it's more than a simple script, it's an entire application. You just plug in its name into the universal entry point pyproject.toml.