nixos-config/configs/pepe/home-assistant/heater-control.nix

{ lib, pkgs, config, ... }: {

  users.users.heater-control = { };

  systemd.services.heater-control = {
    enable = true;
    wantedBy = [ "multi-user.target" ];

    serviceConfig = { User = "heater-control"; };
    script =
      let myPython = pkgs.python3.withPackages (ps: with ps; [ paho-mqtt ]);
      in ''
        cd ${<mqtt>}
        ${myPython}/bin/python heater.py
      '';
  };
}