I have this flake.nix

{
  description = "dspy test";

  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  inputs.flake-utils.url = "github:numtide/flake-utils";
  inputs.dspy = {
    url = "github:stanfordnlp/dspy";
    flake = false;
  };


  outputs = { self, nixpkgs, flake-utils,dspy }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
        python = pkgs.python312;
        python_packages = python.withPackages(ps: with ps;[
          ipython
          matplotlib
          pandas
          pip
          dspy
        ]);

        myDevTools = [
          python_packages
        ];
      in {
        devShells.default = pkgs.mkShell {
          buildInputs = myDevTools;          
        };
      });
}

I use the command nix develop to create a virtual environment. I open a python terminal. I enter import dspi and it works.

But I don’t understand why it works.

THe documentation only say that:

Repositories that don’t contain a flake.nix can also be used as
inputs, by setting the input’s flake attribute to false:

but it doesn’t explain how it is built