gh-115754: Add Py_GetConstant() function (#116883)

Add Py_GetConstant() and Py_GetConstantBorrowed() functions.

In the limited C API version 3.13, getting Py_None, Py_False,
Py_True, Py_Ellipsis and Py_NotImplemented singletons is now
implemented as function calls at the stable ABI level to hide
implementation details. Getting these constants still return borrowed
references.

Add _testlimitedcapi/object.c and test_capi/test_object.py to test
Py_GetConstant() and Py_GetConstantBorrowed() functions.

Modules/Setup.stdlib.in

@@ -163,7 +163,7 @@
 @MODULE__TESTBUFFER_TRUE@_testbuffer _testbuffer.c
 @MODULE__TESTINTERNALCAPI_TRUE@_testinternalcapi _testinternalcapi.c _testinternalcapi/test_lock.c _testinternalcapi/pytime.c _testinternalcapi/set.c _testinternalcapi/test_critical_sections.c
 @MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/heaptype.c _testcapi/abstract.c _testcapi/unicode.c _testcapi/dict.c _testcapi/set.c _testcapi/list.c _testcapi/tuple.c _testcapi/getargs.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/complex.c _testcapi/numbers.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c _testcapi/buffer.c _testcapi/pyatomic.c _testcapi/file.c _testcapi/codec.c _testcapi/immortal.c _testcapi/gc.c _testcapi/hash.c _testcapi/time.c
-@MODULE__TESTLIMITEDCAPI_TRUE@_testlimitedcapi _testlimitedcapi.c _testlimitedcapi/abstract.c _testlimitedcapi/bytearray.c _testlimitedcapi/bytes.c _testlimitedcapi/complex.c _testlimitedcapi/dict.c _testlimitedcapi/float.c _testlimitedcapi/heaptype_relative.c _testlimitedcapi/list.c _testlimitedcapi/long.c _testlimitedcapi/pyos.c _testlimitedcapi/set.c _testlimitedcapi/sys.c _testlimitedcapi/unicode.c _testlimitedcapi/vectorcall_limited.c
+@MODULE__TESTLIMITEDCAPI_TRUE@_testlimitedcapi _testlimitedcapi.c _testlimitedcapi/abstract.c _testlimitedcapi/bytearray.c _testlimitedcapi/bytes.c _testlimitedcapi/complex.c _testlimitedcapi/dict.c _testlimitedcapi/float.c _testlimitedcapi/heaptype_relative.c _testlimitedcapi/list.c _testlimitedcapi/long.c _testlimitedcapi/object.c _testlimitedcapi/pyos.c _testlimitedcapi/set.c _testlimitedcapi/sys.c _testlimitedcapi/unicode.c _testlimitedcapi/vectorcall_limited.c
 @MODULE__TESTCLINIC_TRUE@_testclinic _testclinic.c
 @MODULE__TESTCLINIC_LIMITED_TRUE@_testclinic_limited _testclinic_limited.c
 

Modules/_testlimitedcapi.c

@@ -53,6 +53,9 @@ PyInit__testlimitedcapi(void)
     if (_PyTestLimitedCAPI_Init_Long(mod) < 0) {
         return NULL;
     }
+    if (_PyTestLimitedCAPI_Init_Object(mod) < 0) {
+        return NULL;
+    }
     if (_PyTestLimitedCAPI_Init_PyOS(mod) < 0) {
         return NULL;
     }

Modules/_testlimitedcapi/object.c

@@ -0,0 +1,80 @@
+// Need limited C API version 3.13 for Py_GetConstant()
+#include "pyconfig.h"   // Py_GIL_DISABLED
+#if !defined(Py_GIL_DISABLED) && !defined(Py_LIMITED_API )
+#  define Py_LIMITED_API 0x030d0000
+#endif
+
+#include "parts.h"
+#include "util.h"
+
+
+/* Test Py_GetConstant() */
+static PyObject *
+get_constant(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    int constant_id;
+    if (!PyArg_ParseTuple(args, "i", &constant_id)) {
+        return NULL;
+    }
+
+    PyObject *obj = Py_GetConstant(constant_id);
+    if (obj == NULL) {
+        assert(PyErr_Occurred());
+        return NULL;
+    }
+    return obj;
+}
+
+
+/* Test Py_GetConstantBorrowed() */
+static PyObject *
+get_constant_borrowed(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    int constant_id;
+    if (!PyArg_ParseTuple(args, "i", &constant_id)) {
+        return NULL;
+    }
+
+    PyObject *obj = Py_GetConstantBorrowed(constant_id);
+    if (obj == NULL) {
+        assert(PyErr_Occurred());
+        return NULL;
+    }
+    return Py_NewRef(obj);
+}
+
+
+/* Test constants */
+static PyObject *
+test_constants(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    // Test that implementation of constants in the limited C API:
+    // check that the C code compiles.
+    //
+    // Test also that constants and Py_GetConstant() return the same
+    // objects.
+    assert(Py_None == Py_GetConstant(Py_CONSTANT_NONE));
+    assert(Py_False == Py_GetConstant(Py_CONSTANT_FALSE));
+    assert(Py_True == Py_GetConstant(Py_CONSTANT_TRUE));
+    assert(Py_Ellipsis == Py_GetConstant(Py_CONSTANT_ELLIPSIS));
+    assert(Py_NotImplemented == Py_GetConstant(Py_CONSTANT_NOT_IMPLEMENTED));
+    // Other constants are tested in test_capi.test_object
+    Py_RETURN_NONE;
+}
+
+static PyMethodDef test_methods[] = {
+    {"get_constant", get_constant, METH_VARARGS},
+    {"get_constant_borrowed", get_constant_borrowed, METH_VARARGS},
+    {"test_constants", test_constants, METH_NOARGS},
+    {NULL},
+};
+
+int
+_PyTestLimitedCAPI_Init_Object(PyObject *m)
+{
+    if (PyModule_AddFunctions(m, test_methods) < 0) {
+        return -1;
+    }
+
+    return 0;
+}

Modules/_testlimitedcapi/parts.h

@@ -29,6 +29,7 @@ int _PyTestLimitedCAPI_Init_Complex(PyObject *module);
 int _PyTestLimitedCAPI_Init_Dict(PyObject *module);
 int _PyTestLimitedCAPI_Init_Float(PyObject *module);
 int _PyTestLimitedCAPI_Init_HeaptypeRelative(PyObject *module);
+int _PyTestLimitedCAPI_Init_Object(PyObject *module);
 int _PyTestLimitedCAPI_Init_List(PyObject *module);
 int _PyTestLimitedCAPI_Init_Long(PyObject *module);
 int _PyTestLimitedCAPI_Init_PyOS(PyObject *module);