python3: Fixing excessive memory usage for overriden python

An attempt to address #338231. The current release of nixpkgs takes over
20GB of memory to evaluate python when the interpreter is overriden.
This adds memoization to the spliced python packages to avoid
evaluating all the commbinations of overrides.

pkgs/development/interpreters/python/cpython/default.nix

@@ -55,6 +55,7 @@
   pkgsBuildTarget,
   pkgsHostHost,
   pkgsTargetTarget,
+  __splices ? { },
 
   # build customization
   sourceVersion,
@@ -145,10 +146,28 @@ let
       # When we override the interpreter we also need to override the spliced versions of the interpreter
       # bluez is excluded manually to break an infinite recursion.
       inputs' = lib.filterAttrs (n: v: n != "bluez" && n != "passthruFun" && !lib.isDerivation v) inputs;
+      # Memoization of the splices to avoid re-evaluating this function for all combinations of splices e.g.
+      # python3.pythonOnBuildForHost.pythonOnBuildForTarget == python3.pythonOnBuildForTarget by consuming
+      # __splices as an arg and using the cache if populated.
+      splices = {
+        pythonOnBuildForBuild = override pkgsBuildBuild.${pythonAttr};
+        pythonOnBuildForHost = override pkgsBuildHost.${pythonAttr};
+        pythonOnBuildForTarget = override pkgsBuildTarget.${pythonAttr};
+        pythonOnHostForHost = override pkgsHostHost.${pythonAttr};
+        pythonOnTargetForTarget = lib.optionalAttrs (lib.hasAttr pythonAttr pkgsTargetTarget) (
+          override pkgsTargetTarget.${pythonAttr}
+        );
+      } // __splices;
       override =
         attr:
         let
-          python = attr.override (inputs' // { self = python; });
+          python = attr.override (
+            inputs'
+            // {
+              self = python;
+              __splices = splices;
+            }
+          );
         in
         python;
     in
@@ -160,13 +179,13 @@ let
       pythonVersion = with sourceVersion; "${major}.${minor}";
       sitePackages = "lib/${libPrefix}/site-packages";
       inherit hasDistutilsCxxPatch pythonAttr;
-      pythonOnBuildForBuild = override pkgsBuildBuild.${pythonAttr};
-      pythonOnBuildForHost = override pkgsBuildHost.${pythonAttr};
-      pythonOnBuildForTarget = override pkgsBuildTarget.${pythonAttr};
-      pythonOnHostForHost = override pkgsHostHost.${pythonAttr};
-      pythonOnTargetForTarget = lib.optionalAttrs (lib.hasAttr pythonAttr pkgsTargetTarget) (
-        override pkgsTargetTarget.${pythonAttr}
-      );
+      inherit (splices)
+        pythonOnBuildForBuild
+        pythonOnBuildForHost
+        pythonOnBuildForTarget
+        pythonOnHostForHost
+        pythonOnTargetForTarget
+        ;
     };
 
   version = with sourceVersion; "${major}.${minor}.${patch}${suffix}";