add generic implementation of a __dict__ descriptor for C types

Doc/c-api/object.rst

@@ -101,6 +101,18 @@ Object Protocol
    This is the equivalent of the Python statement ``del o.attr_name``.
 
 
+.. c:function:: PyObject* PyType_GenericGetDict(PyObject *o, void *context)
+
+   A generic implementation for the getter of a ``__dict__`` descriptor. It
+   creates the dictionary if necessary.
+
+
+.. c:function:: int PyType_GenericSetDict(PyObject *o, void *context)
+
+   A generic implementation for the setter of a ``__dict__`` descriptor. This
+   implementation does not allow the dictionary to be deleted.
+
+
 .. c:function:: PyObject* PyObject_RichCompare(PyObject *o1, PyObject *o2, int opid)
 
    Compare the values of *o1* and *o2* using the operation specified by *opid*,

Doc/c-api/type.rst

@@ -77,7 +77,6 @@ Type Objects
 
    XXX: Document.
 
-
 .. c:function:: int PyType_Ready(PyTypeObject *type)
 
    Finalize a type object.  This should be called on all type objects to finish

Include/object.h

@@ -516,6 +516,8 @@ PyAPI_FUNC(PyObject *) _PyObject_NextNotImplemented(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_GenericGetAttr(PyObject *, PyObject *);
 PyAPI_FUNC(int) PyObject_GenericSetAttr(PyObject *,
                                               PyObject *, PyObject *);
+PyAPI_FUNC(PyObject *) PyObject_GenericGetDict(PyObject *, void *);
+PyAPI_FUNC(int) PyObject_GenericSetDict(PyObject *, PyObject *, void *);
 PyAPI_FUNC(Py_hash_t) PyObject_Hash(PyObject *);
 PyAPI_FUNC(Py_hash_t) PyObject_HashNotImplemented(PyObject *);
 PyAPI_FUNC(int) PyObject_IsTrue(PyObject *);

Misc/NEWS

@@ -2253,6 +2253,10 @@ Tests
 C-API
 -----
 
+- Add PyObject_GenericGetDict and PyObject_GeneriSetDict. They are generic
+  implementations for the getter and setter of a ``__dict__`` descriptor of C
+  types.
+
 - Issue #13727: Add 3 macros to access PyDateTime_Delta members:
   PyDateTime_DELTA_GET_DAYS, PyDateTime_DELTA_GET_SECONDS,
   PyDateTime_DELTA_GET_MICROSECONDS.

Objects/object.c

@@ -1210,6 +1210,48 @@ PyObject_GenericSetAttr(PyObject *obj, PyObject *name, PyObject *value)
     return _PyObject_GenericSetAttrWithDict(obj, name, value, NULL);
 }
 
+PyObject *
+PyObject_GenericGetDict(PyObject *obj, void *context)
+{
+    PyObject *dict, **dictptr = _PyObject_GetDictPtr(obj);
+    if (dictptr == NULL) {
+        PyErr_SetString(PyExc_AttributeError,
+                        "This object has no __dict__");
+        return NULL;
+    }
+    dict = *dictptr;
+    if (dict == NULL)
+        *dictptr = dict = PyDict_New();
+    Py_XINCREF(dict);
+    return dict;
+}
+
+int
+PyObject_GenericSetDict(PyObject *obj, PyObject *value, void *context)
+{
+    PyObject *dict, **dictptr = _PyObject_GetDictPtr(obj);
+    if (dictptr == NULL) {
+        PyErr_SetString(PyExc_AttributeError,
+                        "This object has no __dict__");
+        return -1;
+    }
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError, "cannot delete __dict__");
+        return -1;
+    }
+    if (!PyDict_Check(value)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__dict__ must be set to a dictionary, "
+                     "not a '%.200s'", Py_TYPE(value)->tp_name);
+        return -1;
+    }
+    dict = *dictptr;
+    Py_XINCREF(value);
+    *dictptr = value;
+    Py_XDECREF(dict);
+    return 0;
+}
+
 
 /* Test a value used as condition, e.g., in a for or if statement.
    Return -1 if an error occurred */

Objects/typeobject.c

@@ -1759,8 +1759,6 @@ raise_dict_descr_error(PyObject *obj)
 static PyObject *
 subtype_dict(PyObject *obj, void *context)
 {
-    PyObject **dictptr;
-    PyObject *dict;
     PyTypeObject *base;
 
     base = get_builtin_base_with_dict(Py_TYPE(obj));
@@ -1778,25 +1776,13 @@ subtype_dict(PyObject *obj, void *context)
         }
         return func(descr, obj, (PyObject *)(Py_TYPE(obj)));
     }
-
-    dictptr = _PyObject_GetDictPtr(obj);
-    if (dictptr == NULL) {
-        PyErr_SetString(PyExc_AttributeError,
-                        "This object has no __dict__");
-        return NULL;
-    }
-    dict = *dictptr;
-    if (dict == NULL)
-        *dictptr = dict = PyDict_New();
-    Py_XINCREF(dict);
-    return dict;
+    return PyObject_GenericGetDict(obj, context);
 }
 
 static int
 subtype_setdict(PyObject *obj, PyObject *value, void *context)
 {
-    PyObject **dictptr;
-    PyObject *dict;
+    PyObject *dict, **dictptr;
     PyTypeObject *base;
 
     base = get_builtin_base_with_dict(Py_TYPE(obj));
@@ -1814,14 +1800,14 @@ subtype_setdict(PyObject *obj, PyObject *value, void *context)
         }
         return func(descr, obj, value);
     }
-
+    /* Almost like PyObject_GenericSetDict, but allow __dict__ to be deleted. */
     dictptr = _PyObject_GetDictPtr(obj);
     if (dictptr == NULL) {
         PyErr_SetString(PyExc_AttributeError,
                         "This object has no __dict__");
         return -1;
     }
-    if (value != NULL && !PyDict_Check(value)) {
+    if (!PyDict_Check(value)) {
         PyErr_Format(PyExc_TypeError,
                      "__dict__ must be set to a dictionary, "
                      "not a '%.200s'", Py_TYPE(value)->tp_name);