Add PyObject_GenericHash() function

Also, make Py_HashPointer() available in the limited C API.

Author: vstinner

docs/api.rst

@@ -465,7 +465,9 @@ Python 3.13
 
    See `Py_HashPointer() documentation <https://docs.python.org/dev/c-api/hash.html#c.Py_HashPointer>`__.
 
-   Not available on the limited C API.
+.. c:function:: Py_hash_t PyObject_GenericHash(PyObject *obj)
+
+   See `PyObject_GenericHash() documentation <https://docs.python.org/dev/c-api/hash.html#c.PyObject_GenericHash>`__.
 
 .. c:type:: PyTime_t
 
@@ -533,7 +535,6 @@ Not supported:
 * ``PyLong_AsNativeBytes()``
 * ``PyLong_FromNativeBytes()``
 * ``PyLong_FromUnsignedNativeBytes()``
-* ``PyObject_GenericHash()``.
 * ``PySys_Audit()``.
 * ``PySys_AuditTuple()``.
 

docs/changelog.rst

@@ -1,7 +1,8 @@
 Changelog
 =========
 
-* 2026-10-10: Add ``PyUnicodeWriter_WriteUCS4()`` function.
+* 2026-10-10: Add ``PyObject_GenericHash()`` and
+  ``PyUnicodeWriter_WriteUCS4()`` functions.
 * 2026-10-08: Add support for the limited C API 3.2.
 * 2026-10-08: Add functions:
 

pythoncapi_compat.h

@@ -1236,14 +1236,39 @@ PyDict_PopString(PyObject *dict, const char *key, PyObject **result)
 
 
 // gh-111545 added Py_HashPointer() to Python 3.13
-#if PY_VERSION_HEX < _Py_PACK_VERSION(3, 13) && !defined(Py_LIMITED_API)
+#if PY_VERSION_HEX < _Py_PACK_VERSION(3, 13) || defined(Py_LIMITED_API)
 static inline Py_hash_t Py_HashPointer(const void *ptr)
 {
-#if PY_VERSION_HEX >= _Py_PACK_VERSION(3, 9) && !defined(PYPY_VERSION)
+#if PY_VERSION_HEX >= _Py_PACK_VERSION(3, 9) && !defined(PYPY_VERSION) && !defined(Py_LIMITED_API)
     return _Py_HashPointer(ptr);
-#else
+#elif !defined(Py_LIMITED_API)
+    // PyPy _Py_HashPointer() argument type is "void*", not "const void*"
     return _Py_HashPointer(_Py_CAST(void*, ptr));
+#else
+    // Limited C API implementation
+    uintptr_t x = (uintptr_t)ptr;
+    Py_BUILD_ASSERT(sizeof(x) == sizeof(ptr));
+
+    // Bottom 3 or 4 bits are likely to be 0; rotate x by 4 to the right
+    // to avoid excessive hash collisions for dicts and sets.
+    x = (x >> 4) | (x << (8 * sizeof(uintptr_t) - 4));
+
+    if (x == (uintptr_t)-1) {
+        x = (uintptr_t)-2;
+    }
+
+    Py_BUILD_ASSERT(sizeof(x) == sizeof(Py_hash_t));
+    return (Py_hash_t)x;
+#endif
+}
 #endif
+
+
+#if PY_VERSION_HEX < _Py_PACK_VERSION(3, 13) || defined(Py_LIMITED_API)
+Py_hash_t
+PyObject_GenericHash(PyObject *obj)
+{
+    return Py_HashPointer(obj);
 }
 #endif
 

tests/test_pythoncapi_compat.py

@@ -173,7 +173,7 @@ def run_tests(module_name, std):
             except AttributeError:
                 pass
             else:
-                if attr in ("PY_VERSION_HEX", "PYPY_VERSION_NUM"):
+                if attr in ('PY_VERSION_HEX', 'Py_LIMITED_API', 'PYPY_VERSION_NUM'):
                     value = "0x%x" % value
                 print("%s: %s" % (attr, value))
                 empty_line = True

tests/test_pythoncapi_compat_cext.c

@@ -1762,6 +1762,22 @@ test_hash(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
         Py_DECREF(abc);
     }
 
+    // Test PyObject_GenericHash()
+    {
+        PyObject *object_type = PyImport_ImportModuleAttrString("builtins", "object");
+        if (object_type == NULL) {
+            return NULL;
+        }
+        PyObject *object = PyObject_CallNoArgs(object_type);
+        Py_DECREF(object_type);
+        if (object == NULL) {
+            return NULL;
+        }
+
+        assert(PyObject_GenericHash(object) == Py_HashPointer(object));
+        Py_DECREF(object);
+    }
+
     Py_RETURN_NONE;
 }