separation from main pkgs script

custom/pkgs/by-category/default.nix

@@ -0,0 +1,25 @@
+{
+  pkgs,
+  prev,
+  lib,
+  packagesFromTree,
+}:
+let
+  categoryFolders = {
+    python3Packages = ./python3Packages;
+  };
+
+  pyExts =
+    if categoryFolders ? python3Packages && builtins.pathExists categoryFolders.python3Packages then
+      [
+        (pyFinal: _pyPrev: packagesFromTree categoryFolders.python3Packages pyFinal.callPackage)
+      ]
+    else
+      [ ];
+in
+{
+  pythonPackagesExtensions = (prev.pythonPackagesExtensions or [ ]) ++ pyExts;
+
+  # optional: expose mapping for debugging
+  # inherit categoryFolders;
+}

custom/pkgs/default.nix

@@ -5,7 +5,6 @@
   ...
 }:
 let
-
   dirs = path: lib.attrNames (lib.filterAttrs (_: t: t == "directory") (builtins.readDir path));
 
   packagesFromTree =
@@ -29,21 +28,16 @@ let
       acc // lib.foldl' (a: n: a // addOne n) { } names
     ) { } (dirs base);
 
-  byName = ./by-name;
-  # topLevel = if builtins.pathExists byName then packagesFromTree byName pkgs.callPackage else { };
+  byNamePkgs =
+    if builtins.pathExists ./by-name then packagesFromTree ./by-name pkgs.callPackage else { };
 
-  byPy3 = ./by-category/python3Packages;
-  # pyExts =
-  #   if builtins.pathExists byPy3 then
-  #     [
-  #       (pyFinal: _pyPrev: packagesFromTree byPy3 pyFinal.callPackage)
-  #     ]
-  #   else
-  #     [ ];
+  byCategoryPkgs = import ./by-category {
+    inherit
+      pkgs
+      prev
+      lib
+      packagesFromTree
+      ;
+  };
 in
-(packagesFromTree byName pkgs.callPackage)
-// {
-  pythonPackagesExtensions = (prev.pythonPackagesExtensions or [ ]) ++ [
-    (pyFinal: _pyPrev: packagesFromTree byPy3 pyFinal.callPackage)
-  ];
-}
+byNamePkgs // byCategoryPkgs