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.

1 files changed, 51 insertions, 0 deletions

diff --git a/Objects/object.c b/Objects/object.c
index fcb8cf4..0d03292 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -14,6 +14,7 @@
 #include "pycore_memoryobject.h"  // _PyManagedBuffer_Type
 #include "pycore_namespace.h"     // _PyNamespace_Type
 #include "pycore_object.h"        // PyAPI_DATA() _Py_SwappedOp definition
+#include "pycore_long.h"          // _PyLong_GetZero()
 #include "pycore_optimizer.h"     // _PyUOpExecutor_Type, _PyUOpOptimizer_Type, ...
 #include "pycore_pyerrors.h"      // _PyErr_Occurred()
 #include "pycore_pymem.h"         // _PyMem_IsPtrFreed()
@@ -2991,3 +2992,53 @@ _Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt)
 {
     Py_SET_REFCNT(ob, refcnt);
 }
+
+
+static PyObject* constants[] = {
+    &_Py_NoneStruct,                   // Py_CONSTANT_NONE
+    (PyObject*)(&_Py_FalseStruct),     // Py_CONSTANT_FALSE
+    (PyObject*)(&_Py_TrueStruct),      // Py_CONSTANT_TRUE
+    &_Py_EllipsisObject,               // Py_CONSTANT_ELLIPSIS
+    &_Py_NotImplementedStruct,         // Py_CONSTANT_NOT_IMPLEMENTED
+    NULL,  // Py_CONSTANT_ZERO
+    NULL,  // Py_CONSTANT_ONE
+    NULL,  // Py_CONSTANT_EMPTY_STR
+    NULL,  // Py_CONSTANT_EMPTY_BYTES
+    NULL,  // Py_CONSTANT_EMPTY_TUPLE
+};
+
+void
+_Py_GetConstant_Init(void)
+{
+    constants[Py_CONSTANT_ZERO] = _PyLong_GetZero();
+    constants[Py_CONSTANT_ONE] = _PyLong_GetOne();
+    constants[Py_CONSTANT_EMPTY_STR] = PyUnicode_New(0, 0);
+    constants[Py_CONSTANT_EMPTY_BYTES] = PyBytes_FromStringAndSize(NULL, 0);
+    constants[Py_CONSTANT_EMPTY_TUPLE] = PyTuple_New(0);
+#ifndef NDEBUG
+    for (size_t i=0; i < Py_ARRAY_LENGTH(constants); i++) {
+        assert(constants[i] != NULL);
+        assert(_Py_IsImmortal(constants[i]));
+    }
+#endif
+}
+
+PyObject*
+Py_GetConstant(unsigned int constant_id)
+{
+    if (constant_id < Py_ARRAY_LENGTH(constants)) {
+        return constants[constant_id];
+    }
+    else {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+}
+
+
+PyObject*
+Py_GetConstantBorrowed(unsigned int constant_id)
+{
+    // All constants are immortal
+    return Py_GetConstant(constant_id);
+}