You don't strictly even need a `shell.nix`, you can just name packages verbatim on the command line a la `nix-shell -p python39.pkgs.ipython -p python39.pkgs.nu

You don't strictly even need a `shell.nix`, you can just name packages verbatim on the command line a la `nix-shell -p python39.pkgs.ipython -p python39.pkgs.numpy --pure` and you're away.