It depends. If the dependency is in nixpkgs (Nix' own package repository), someone will have done the work of figuring out the dependencies properly and you can just use that. If it's not, you can either do that yourself and pin particular versions, or you can integrate with a tool like Poetry: Poetry could generate a lockfile for you that you then reference from Nix to get the versions of Python packages. You'd still need to specify any native dependencies manually, though.