gh-118965: Share NotImplemented in subinterpreters

Lib/test/test__interpreters.py

@@ -99,6 +99,7 @@ def test_default_shareables(self):
         shareables = [
                 # singletons
                 None,
+                NotImplemented,
                 # builtin objects
                 b'spam',
                 'spam',
@@ -126,7 +127,6 @@ class SubBytes(bytes):
 
         not_shareables = [
                 # singletons
-                NotImplemented,
                 ...,
                 # builtin types and objects
                 type,
@@ -156,13 +156,14 @@ def _assert_values(self, values):
                 self.assertIs(type(got), type(obj))
 
     def test_singletons(self):
-        for obj in [None]:
+        for obj in [None, NotImplemented]:
             with self.subTest(obj):
                 xid = _testinternalcapi.get_crossinterp_data(obj)
                 got = _testinternalcapi.restore_crossinterp_data(xid)
 
                 # XXX What about between interpreters?
                 self.assertIs(got, obj)
+                self.assertIs(type(got), type(obj))
 
     def test_types(self):
         self._assert_values([

Lib/test/test_interpreters/test_api.py

@@ -1101,6 +1101,7 @@ def test_default_shareables(self):
         shareables = [
                 # singletons
                 None,
+                NotImplemented,
                 # builtin objects
                 b'spam',
                 'spam',
@@ -1129,7 +1130,6 @@ class SubBytes(bytes):
 
         not_shareables = [
                 # singletons
-                NotImplemented,
                 ...,
                 # builtin types and objects
                 type,

Misc/NEWS.d/next/Core_and_Builtins/2024-05-12-15-19-40.gh-issue-118965.Lr3qAz.rst

@@ -0,0 +1 @@
+Added support for sharing :data:`NotImplemented` singleton between subinterpreters.

Python/crossinterp_data_lookup.h

@@ -438,6 +438,24 @@
     return 0;
 }
 
+// NotImplemented
+
+static PyObject *
+_new_notimplemented_object(_PyCrossInterpreterData *data)
+{
+    assert(_Py_IsImmortal(Py_NotImplemented));
+    return Py_NotImplemented;
+}
+
+static int
+_notimplemented_shared(PyThreadState *tstate, PyObject *obj,
+                       _PyCrossInterpreterData *data)
+{
+    _PyCrossInterpreterData_Init(data, tstate->interp, NULL, NULL,
+            _new_notimplemented_object);
+    return 0;
+}
+
 // bool
 
 static PyObject *
@@ -567,6 +585,13 @@
         Py_FatalError("could not register None for cross-interpreter sharing");
     }
 
+    // NotImplemented
+    if (_xidregistry_add_type(xidregistry,
+                              (PyTypeObject *)PyObject_Type(Py_NotImplemented),
+                              _notimplemented_shared) != 0) {
+        Py_FatalError("could not register NotImplemented for cross-interpreter sharing");
+    }
+
     // int
     if (_xidregistry_add_type(xidregistry, &PyLong_Type, _long_shared) != 0) {
         Py_FatalError("could not register int for cross-interpreter sharing");