fix python to allow multiple versions of dependencies
Allowing Multiple Versions of Python Package in PYTHONPATH/nixpkgs
then expensive packages (tensorflow …) can pin all their dependencies (“bottom-up pinning”, inversion of control), and we avoid rebuilds
then nix can get closer to its promise
the current python situation (collisions in site-packages/) is like the FHS situation (collisions in usr/) that nix wants to solve
… and we avoid rebuilds
the other strategy is to make builds cheaper, for example with Incremental builds
but it requires more complexity (normalize sources, store objects, patch objects)