gh-113024: C API: Add PyObject_GenericHash() function (GH-113025)

Author: serhiy-storchaka

Doc/c-api/hash.rst

@@ -82,3 +82,14 @@ See also the :c:member:`PyTypeObject.tp_hash` member and :ref:`numeric-hash`.
    The function cannot fail: it cannot return ``-1``.
 
    .. versionadded:: 3.13
+
+.. c:function:: Py_hash_t PyObject_GenericHash(PyObject *obj)
+
+   Generic hashing function that is meant to be put into a type
+   object's ``tp_hash`` slot.
+   Its result only depends on the object's identity.
+
+   .. impl-detail::
+      In CPython, it is equivalent to :c:func:`Py_HashPointer`.
+
+   .. versionadded:: 3.13

Doc/c-api/typeobj.rst

@@ -883,6 +883,10 @@ and :c:data:`PyType_Type` effectively act as defaults.)
    :c:member:`~PyTypeObject.tp_richcompare` and :c:member:`~PyTypeObject.tp_hash`, when the subtype's
    :c:member:`~PyTypeObject.tp_richcompare` and :c:member:`~PyTypeObject.tp_hash` are both ``NULL``.
 
+   **Default:**
+
+   :c:data:`PyBaseObject_Type` uses :c:func:`PyObject_GenericHash`.
+
 
 .. c:member:: ternaryfunc PyTypeObject.tp_call
 

Doc/whatsnew/3.13.rst

@@ -1702,6 +1702,10 @@ New Features
 * Add :c:func:`Py_HashPointer` function to hash a pointer.
   (Contributed by Victor Stinner in :gh:`111545`.)
 
+* Add :c:func:`PyObject_GenericHash` function that implements the default
+  hashing function of a Python object.
+  (Contributed by Serhiy Storchaka in :gh:`113024`.)
+
 * Add PyTime C API:
 
   * :c:type:`PyTime_t` type.

Include/cpython/pyhash.h

@@ -43,3 +43,4 @@ typedef struct {
 PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
 
 PyAPI_FUNC(Py_hash_t) Py_HashPointer(const void *ptr);
+PyAPI_FUNC(Py_hash_t) PyObject_GenericHash(PyObject *);

Lib/test/test_capi/test_abstract.py

@@ -1001,6 +1001,12 @@ def test_number_check(self):
         self.assertTrue(number_check(0.5))
         self.assertFalse(number_check("1 + 1j"))
 
+    def test_object_generichash(self):
+        # Test PyObject_GenericHash()
+        generichash = _testcapi.object_generichash
+        for obj in object(), 1, 'string', []:
+            self.assertEqual(generichash(obj), object.__hash__(obj))
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C API/2023-12-12-19-48-31.gh-issue-113024.rXcQs7.rst

@@ -0,0 +1 @@
+Add :c:func:`PyObject_GenericHash` function.

Modules/_decimal/_decimal.c

@@ -4780,7 +4780,7 @@ _dec_hash(PyDecObject *v)
             return -1;
         }
         else if (mpd_isnan(MPD(v))) {
-            return _Py_HashPointer(v);
+            return PyObject_GenericHash((PyObject *)v);
         }
         else {
             return py_hash_inf * mpd_arith_sign(MPD(v));

Modules/_testcapi/hash.c

@@ -59,9 +59,20 @@ hash_pointer(PyObject *Py_UNUSED(module), PyObject *arg)
 }
 
 
+static PyObject *
+object_generichash(PyObject *Py_UNUSED(module), PyObject *arg)
+{
+    NULLABLE(arg);
+    Py_hash_t hash = PyObject_GenericHash(arg);
+    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
+    return PyLong_FromLongLong(hash);
+}
+
+
 static PyMethodDef test_methods[] = {
     {"hash_getfuncdef", hash_getfuncdef, METH_NOARGS},
     {"hash_pointer", hash_pointer, METH_O},
+    {"object_generichash", object_generichash, METH_O},
     {NULL},
 };
 

Objects/classobject.c

@@ -301,7 +301,7 @@ static Py_hash_t
 method_hash(PyMethodObject *a)
 {
     Py_hash_t x, y;
-    x = _Py_HashPointer(a->im_self);
+    x = PyObject_GenericHash(a->im_self);
     y = PyObject_Hash(a->im_func);
     if (y == -1)
         return -1;

Objects/descrobject.c

@@ -1346,7 +1346,7 @@ wrapper_hash(PyObject *self)
 {
     wrapperobject *wp = (wrapperobject *)self;
     Py_hash_t x, y;
-    x = _Py_HashPointer(wp->self);
+    x = PyObject_GenericHash(wp->self);
     y = _Py_HashPointer(wp->descr);
     x = x ^ y;
     if (x == -1)