{
  description = "Python dev env (NixOS 25.11) using project-local venv";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};

        # Pick your Python version here
        python = pkgs.python311; # or python312, etc.
      in {
        devShells.default = pkgs.mkShell {
          name = "python-venv-shell";

          packages = [
            python
            pkgs.python311Packages.virtualenv  # gives `virtualenv` tool
          ];

          shellHook = ''
            echo "== Nix dev shell (Python $(python --version 2>&1)) =="

            # Create a venv in .venv if it doesn't exist yet
            if [ ! -d ".venv" ]; then
              echo "Creating local virtualenv in .venv ..."
              python -m venv .venv
            fi

            # Activate the venv
            echo "Activating .venv"
            . .venv/bin/activate

            # Tell you what Python/pip you're using
            echo "Using python: $(which python)"
            echo "Using pip:    $(which pip)"

            # Install / update deps from requirements.txt *into .venv*
            if [ -f "requirements.txt" ]; then
              echo "Installing dependencies from requirements.txt into .venv ..."
              pip install -r requirements.txt
            else
              echo "No requirements.txt found in $(pwd)"
            fi

            # Prompt tweak so you can see when venv is active
            export NIX_PROJECT_SHELL="SPARC"
          '';
        };
      });
}