Make the pypy3 kernel build and run

modules/kernels/python/kernel.nix

@@ -32,7 +32,8 @@ common.makeJupyterKernel (
       displayName = displayName;
       language = kernelName;
       argv = [
-        "${python}/bin/python"
+        # PyPy environments name the binary pypy3, so take the path the environment reports.
+        python.interpreter
         "-m"
         "ipykernel"
         "-f"

modules/kernels/python/module.nix

@@ -192,7 +192,27 @@ in
         config.pkgs.callPackage ./. {
           name = "PyPy";
 
-          python3 = lib.getAttr config.kernels.pypy3.python3Package config.pkgs;
+          python3 = (lib.getAttr config.kernels.pypy3.python3Package config.pkgs).override {
+            packageOverrides = pyFinal: pyPrev:
+              # Test suites are the main thing standing between PyPy and a working kernel: a
+              # package unsupported on PyPy throws when its outPath is evaluated even as a mere
+              # check input, and several check inputs are C extensions that don't compile here.
+              (lib.mapAttrs (_name: value:
+                let isPythonPackage = builtins.tryEval (lib.isDerivation value && value ? overridePythonAttrs);
+                in if isPythonPackage.success && isPythonPackage.value
+                   then value.overridePythonAttrs (_: { doCheck = false; })
+                   else value
+              ) pyPrev)
+              // {
+                # On PyPy, pyzmq builds and runs against cffi rather than Cython, but nixpkgs
+                # only wires up the Cython path.
+                pyzmq = pyPrev.pyzmq.overridePythonAttrs (old: {
+                  doCheck = false;
+                  build-system = (old.build-system or []) ++ [ pyFinal.cffi pyFinal.pycparser ];
+                  dependencies = (old.dependencies or []) ++ [ pyFinal.cffi pyFinal.pycparser ];
+                });
+              };
+          };
 
           settings = config.kernels.pypy3;
           settingsSchema = nixosOptionsToSettingsSchema { componentsToDrop = 2; } options.kernels.pypy3;

sample_environments/pypy3.nix

@@ -5,10 +5,5 @@
 codedown.makeEnvironment {
   name = "pypy3";
 
-  # kernels.pypy3.enable = true;
-  # kernels.pypy3.packages = [
-  #   "matplotlib"
-  #   "scipy"
-  #   "rope"
-  # ];
+  kernels.pypy3.enable = true;
 }

tests/app/Spec/Tests/Python.hs

@@ -26,7 +26,7 @@ tests = describe "Python" $ parallel $ do
   tests' ("python3", "python313")
   -- tests' ("python3", "python314")
 
-  -- tests' ("pypy3", "pypy3")
+  pypyTests
 
 tests' :: (Text, Text) -> LanguageSpec
 tests' (kernelName, pythonPackage) = introduceNixEnvironment [kernelSpec kernelName pythonPackage] [] [i|Python (#{pythonPackage})|] $ introduceJupyterRunner $ do
@@ -72,6 +72,26 @@ tests' (kernelName, pythonPackage) = introduceNixEnvironment [kernelSpec kernelN
     assertDiagnosticRanges diagnostics []
 
 
+-- | Nixpkgs carries far less for PyPy than for CPython, so this covers the kernel itself
+-- rather than the package and language server stack the CPython tests exercise.
+pypyTests :: LanguageSpec
+pypyTests = introduceNixEnvironment [pypyKernelSpec] [] "Python (pypy3)" $ introduceJupyterRunner $ do
+  testHasExpectedFields "pypy3"
+
+  testKernelStdout "pypy3" [i|print("hi")|] "hi\n"
+  testKernelStdout "pypy3" [i|import platform; print(platform.python_implementation())|] "PyPy\n"
+
+pypyKernelSpec :: NixKernelSpec
+pypyKernelSpec = NixKernelSpec {
+  nixKernelName = "pypy3"
+  , nixKernelChannel = "codedown"
+  , nixKernelDisplayName = Just "PyPy"
+  , nixKernelPackages = []
+  , nixKernelMeta = Nothing
+  , nixKernelIcon = Nothing
+  , nixKernelExtraConfig = Nothing
+  }
+
 kernelSpec :: Text -> Text -> NixKernelSpec
 kernelSpec kernelName pythonPackage = NixKernelSpec {
   nixKernelName = kernelName