diff --git a/examples/activation/flake.nix b/examples/activation/flake.nix
index cc1fe5c..8ddaf1b 100644
--- a/examples/activation/flake.nix
+++ b/examples/activation/flake.nix
@@ -14,5 +14,7 @@
     kernel-builder.lib.genFlakeOutputs {
       path = ./.;
       rev = self.shortRev or self.dirtyShortRev or self.lastModifiedDate;
+      # Example of adding Python test dependencies directly in the flake
+      pythonTestDeps = [ "pytest-benchmark" ];
     };
 }
diff --git a/flake.nix b/flake.nix
index 3bb51d9..147493f 100644
--- a/flake.nix
+++ b/flake.nix
@@ -54,7 +54,13 @@
           in
           builtins.toJSON (nixpkgs.lib.foldl' (acc: system: acc // buildVariants system) { } systems);
         genFlakeOutputs =
-          { path, rev }:
+          {
+            path,
+            rev,
+            pythonTestDeps ? [ ],
+            pythonDevDeps ? [ ],
+            customPythonPackages ? { },
+          }:
           flake-utils.lib.eachSystem systems (
             system:
             let
@@ -71,10 +77,14 @@
                 devShells = build.torchDevShells {
                   inherit path;
                   rev = revUnderscored;
+                  extraPythonPackages = pythonDevDeps;
+                  inherit customPythonPackages;
                 };
                 testShells = build.torchExtensionShells {
                   inherit path;
                   rev = revUnderscored;
+                  extraPythonPackages = pythonTestDeps;
+                  inherit customPythonPackages;
                 };
               };
               packages = rec {
@@ -182,5 +192,23 @@
     )
     // {
       inherit lib;
+      # Export the helper function
+      mkShellsWithExtraPackages =
+        {
+          pythonTestDeps ? [ ],
+          pythonDevDeps ? [ ],
+        }:
+        {
+          genFlakeOutputs =
+            { path, rev }:
+            lib.genFlakeOutputs {
+              inherit
+                path
+                rev
+                pythonTestDeps
+                pythonDevDeps
+                ;
+            };
+        };
     };
 }
diff --git a/lib/build.nix b/lib/build.nix
index 730ff3d..1907383 100644
--- a/lib/build.nix
+++ b/lib/build.nix
@@ -195,21 +195,56 @@ rec {
   # Get a development shell with the extension in PYTHONPATH. Handy
   # for running tests.
   torchExtensionShells =
-    { path, rev }:
+    {
+      path,
+      rev,
+      extraPythonPackages ? [ ],
+      customPythonPackages ? { },
+    }:
     let
+      buildConfig = readBuildConfig path;
+
       shellForBuildSet =
         { path, rev }:
         buildSet: {
           name = torchBuildVersion buildSet;
           value =
             with buildSet.pkgs;
+            let
+              # Function to resolve nixpkgs packages or custom packages
+              resolvePythonPackage =
+                name:
+                if builtins.hasAttr name customPythonPackages then
+                  python3.pkgs.buildPythonPackage {
+                    pname = name;
+                    version = "custom";
+                    src = customPythonPackages.${name};
+                    doCheck = false;
+                    nativeBuildInputs = [
+                      python3.pkgs.setuptools
+                      python3.pkgs.wheel
+                    ];
+                  }
+                else if lib.hasPrefix "git+" name then
+                  throw "ERROR: Git packages like '${name}' require flake inputs. Add as input: ${name} = { url = \"${lib.removePrefix "git+" name}\"; flake = false; } then use customPythonPackages = { ${name} = ${name}; }."
+                else if builtins.hasAttr name python3.pkgs then
+                  python3.pkgs.${name}
+                else
+                  throw "Python package '${name}' not found in nixpkgs or customPythonPackages";
+
+              # Resolve all packages
+              allPackages = map resolvePythonPackage extraPythonPackages;
+            in
             mkShell {
               buildInputs = [
                 (python3.withPackages (
-                  ps: with ps; [
+                  ps:
+                  with ps;
+                  [
                     buildSet.torch
                     pytest
                   ]
+                  ++ allPackages
                 ))
               ];
               shellHook = ''
@@ -217,12 +252,17 @@ rec {
               '';
             };
         };
-      filteredBuildSets = applicableBuildSets (readBuildConfig path) buildSets;
+      filteredBuildSets = applicableBuildSets buildConfig buildSets;
     in
     builtins.listToAttrs (lib.map (shellForBuildSet { inherit path rev; }) filteredBuildSets);
 
   torchDevShells =
-    { path, rev }:
+    {
+      path,
+      rev,
+      extraPythonPackages ? [ ],
+      customPythonPackages ? { },
+    }:
     let
       shellForBuildSet =
         buildSet:
@@ -239,7 +279,35 @@ rec {
               build2cmake
               kernel-abi-check
             ];
-            buildInputs = with pkgs; [ python3.pkgs.pytest ];
+            buildInputs = with pkgs; [
+              (python3.withPackages (
+                ps:
+                with ps;
+                [
+                  pytest
+                ]
+                ++ (map (
+                  name:
+                  if builtins.hasAttr name customPythonPackages then
+                    python3.pkgs.buildPythonPackage {
+                      pname = name;
+                      version = "custom";
+                      src = customPythonPackages.${name};
+                      doCheck = false;
+                      nativeBuildInputs = [
+                        python3.pkgs.setuptools
+                        python3.pkgs.wheel
+                      ];
+                    }
+                  else if lib.hasPrefix "git+" name then
+                    throw "ERROR: Git packages like '${name}' require flake inputs. Add as input: ${name} = { url = \"${lib.removePrefix "git+" name}\"; flake = false; } then use customPythonPackages = { ${name} = ${name}; }."
+                  else if builtins.hasAttr name python3.pkgs then
+                    python3.pkgs.${name}
+                  else
+                    throw "Python package '${name}' not found in nixpkgs or customPythonPackages"
+                ) extraPythonPackages)
+              ))
+            ];
             inputsFrom = [ (buildTorchExtension buildSet { inherit path rev; }) ];
             env = lib.optionalAttrs rocmSupport {
               PYTORCH_ROCM_ARCH = lib.concatStringsSep ";" buildSet.torch.rocmArchs;