[stabbles]

It's not just that, it's also a filesystem layout issue. If you install everything in `/usr` or `<venv>/lib/pythonX.Y/site-packages` you cannot have two versions / variants of the same package installed concurrently.

For that you need one prefix per installation, which is what Nix, Guix, and Spack do.

[avsm]

The runtime can also use mount namespaces to support concurrent installations. Or, if there is a compilation step, the linker can not expose symbols for clashing libraries and just resolve them within the dependency chain.

The package calculus allows all of these to specified cleanly in a single form.