What is the recommended way to use both nixpkgs stable and unstable / part-specific nixpkgs extension

[onsails]

Before flake-parts, I've ben setting unstable as an overlay

(self: super: { unstable = import inputs.nixpkgs-unstable { inherit system; }; })

I believe the same can be achieved with flake-parts:

  imports = [
    inputs.flake-parts.flakeModules.easyOverlay
  ];

  perSystem = { config, pkgs, final, system, ... }:
    let
      unstable = import inputs.nixpkgs-unstable {
        inherit system;
      };
    in
    {
      overlayAttrs = {
        inherit unstable;
      };
    };

This works well and defined in a special default module I import in every nixosConfiguration.

However, in some parts I need to extend packages specificly for those parts, for example set unfreePredicate – which is ignored apparently because easyOverlay forces it's own nixpkgs configuration. Is it possible to

[roberth]

Is it possible to

I think something went wrong with your post.

However, in some parts I need to extend packages specificly

easyOverlay needs to override pkgs for the purpose of making perSystem behave like an overlay.
Maybe all you really need is to apply an overlay for the flake's own purpose instead of exposing it? In that case a better Nixpkgs module would be sufficient to solve your problem; one with a nixpkgs.overlays option that initializes pkgs with those, instead of the fancy override business to publish rather than consume an overlay.

[stites]

More concretely, I think this suggestion is to package this up in a module like:

{config, lib, pkgs, ...}: {
  sound.enable = true;

  environment = {
    etc."xdg/qutebrowser/config.py".text = ...;
    sessionVariables.BROWSER = "qute";
    systemPackages = with pkgs; [qute qutebrowser mpv youtubeDL rofi];
  };
  nixpkgs.overlays = [
    (final: prev: {
      # wrapper to specify config file
      qute = prev.writeShellScriptBin "qute" ''
        QUTE_DARKMODE_VARIANT=qt_515_2 QT_QPA_PLATFORMTHEME= exec ${final.qutebrowser}/bin/qutebrowser -C /etc/xdg/qutebrowser/config.py "$@"
      '';
    })
  ];
}

and then import the module, right?

[stites]

@onsails, I came across this issue for a different reason than using the pkgs.unstable pattern, but I would like to point out that it seems as if the "flake-parts" way is to avoid this pattern of a global namespace as much as possible.

Some more modular ways to do this seem to be:

• @roberth's suggestion (which I think I gave a concrete example of). This does seem to be a little too much effort for me, personally, when I just want a quick, one-off hack for a tool I have not bought into yet.
• maintain a dedicated pkgs.nix flake, or use something like https://github.com/fufexan/dotfiles/tree/main/pkgs/default.nix which can use packages via a self'.packages property
• Stick to exactly what you are doing, just make sure you place the overlay in the final module you want to work with: https://github.com/fufexan/dotfiles/blob/main/modules/nix.nix#L69-L122

Something I was expecting which did not work, which might also have been your assumption, was:

• defining a flake with a _module.args.pkgs = ... property and importing it at the top level (as in https://github.com/fufexan/dotfiles/blob/main/flake.nix#L13). This doesn't seem to work for me, but take it with a grain of salt, because I'm also not testing out easyOverlay or doing my full due-diligence.