bpo-43916: Use Py_TPFLAGS_DISABLE_NEW flag in hashlib

Doc/c-api/typeobj.rst

@@ -1199,6 +1199,17 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
       .. versionadded:: 3.10
 
+   .. data:: Py_TPFLAGS_DISABLE_NEW
+
+      Disallow creating instances of the type. For example, calling its
+      constructor raises a :exc:`TypeError`.
+
+      **Inheritance:**
+
+      This flag is not inherited.
+
+      .. versionadded:: 3.10
+
 
 .. c:member:: const char* PyTypeObject.tp_doc
 
@@ -2596,7 +2607,8 @@ A type that supports weakrefs, instance dicts, and hashing::
    };
 
 A str subclass that cannot be subclassed and cannot be called
-to create instances (e.g. uses a separate factory func)::
+to create instances (e.g. uses a separate factory func) using
+:c:data:`Py_TPFLAGS_DISABLE_NEW` flag::
 
    typedef struct {
        PyUnicodeObject raw;
@@ -2609,8 +2621,7 @@ to create instances (e.g. uses a separate factory func)::
        .tp_basicsize = sizeof(MyStr),
        .tp_base = NULL,  // set to &PyUnicode_Type in module init
        .tp_doc = "my custom str",
-       .tp_flags = Py_TPFLAGS_DEFAULT,
-       .tp_new = NULL,
+       .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
        .tp_repr = (reprfunc)myobj_repr,
    };
 

Doc/whatsnew/3.10.rst

@@ -1708,6 +1708,10 @@ New Features
   These functions allow to activate, deactivate and query the state of the garbage collector from C code without
   having to import the :mod:`gc` module.
 
+* Add a new :c:data:`Py_TPFLAGS_DISABLE_NEW` type flag to disallow creating
+  instances of a type.
+  (Contributed by Victor Stinner in :issue:`43916`.)
+
 Porting to Python 3.10
 ----------------------
 

Include/object.h

@@ -320,6 +320,10 @@ Code can use PyType_HasFeature(type_ob, flag_value) to test whether the
 given type object has a specified feature.
 */
 
+/* Disallow creating instances of the type. For example, calling its
+ * constructor raises a TypeError */
+#define Py_TPFLAGS_DISABLE_NEW (1UL << 7)
+
 /* Set if the type object is immutable: type attributes cannot be set nor deleted */
 #define Py_TPFLAGS_IMMUTABLETYPE (1UL << 8)
 

Lib/test/test_hashlib.py

@@ -905,11 +905,11 @@ def test_get_fips_mode(self):
     def test_internal_types(self):
         # internal types like _hashlib.HASH are not constructable
         with self.assertRaisesRegex(
-            TypeError, "cannot create 'HASH' instance"
+            TypeError, "cannot create '_hashlib.HASH' instance"
         ):
             HASH()
         with self.assertRaisesRegex(
-            TypeError, "cannot create 'HASHXOF' instance"
+            TypeError, "cannot create '_hashlib.HASHXOF' instance"
         ):
             HASHXOF()
 

Lib/test/test_hmac.py

@@ -440,7 +440,7 @@ def test_withmodule(self):
     def test_internal_types(self):
         # internal types like _hashlib.C_HMAC are not constructable
         with self.assertRaisesRegex(
-            TypeError, "cannot create 'HMAC' instance"
+            TypeError, "cannot create '_hashlib.HMAC' instance"
         ):
             C_HMAC()
 

Lib/test/test_sys.py

@@ -605,11 +605,12 @@ def test_sys_flags(self):
     def assert_raise_on_new_sys_type(self, sys_attr):
         # Users are intentionally prevented from creating new instances of
         # sys.flags, sys.version_info, and sys.getwindowsversion.
+        arg = sys_attr
         attr_type = type(sys_attr)
         with self.assertRaises(TypeError):
-            attr_type()
+            attr_type(arg)
         with self.assertRaises(TypeError):
-            attr_type.__new__(attr_type)
+            attr_type.__new__(attr_type, arg)
 
     def test_sys_flags_no_instantiation(self):
         self.assert_raise_on_new_sys_type(sys.flags)

Misc/NEWS.d/next/C API/2021-04-29-17-35-48.bpo-43916.wvWt23.rst

@@ -0,0 +1,2 @@
+Add a new :c:data:`Py_TPFLAGS_DISABLE_NEW` type flag to disallow creating
+instances of a type. Patch by Victor Stinner.

Modules/_curses_panel.c

@@ -520,7 +520,7 @@ static PyType_Slot PyCursesPanel_Type_slots[] = {
 static PyType_Spec PyCursesPanel_Type_spec = {
     .name = "_curses_panel.panel",
     .basicsize = sizeof(PyCursesPanelObject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
     .slots = PyCursesPanel_Type_slots
 };
 
@@ -656,7 +656,6 @@ _curses_panel_exec(PyObject *mod)
     if (state->PyCursesPanel_Type == NULL) {
         return -1;
     }
-    ((PyTypeObject *)state->PyCursesPanel_Type)->tp_new = NULL;
 
     if (PyModule_AddType(mod, state->PyCursesPanel_Type) < 0) {
         return -1;

Modules/_cursesmodule.c

@@ -4794,24 +4794,18 @@ PyInit__curses(void)
     /* ncurses_version */
     if (NcursesVersionType.tp_name == NULL) {
         if (PyStructSequence_InitType2(&NcursesVersionType,
-                                       &ncurses_version_desc) < 0)
+                                       &ncurses_version_desc) < 0) {
             return NULL;
+        }
+        /* prevent user from creating new instances */
+        NcursesVersionType.tp_flags |= Py_TPFLAGS_DISABLE_NEW;
     }
     v = make_ncurses_version();
     if (v == NULL) {
         return NULL;
     }
     PyDict_SetItemString(d, "ncurses_version", v);
     Py_DECREF(v);
-
-    /* prevent user from creating new instances */
-    NcursesVersionType.tp_init = NULL;
-    NcursesVersionType.tp_new = NULL;
-    if (PyDict_DelItemString(NcursesVersionType.tp_dict, "__new__") < 0 &&
-        PyErr_ExceptionMatches(PyExc_KeyError))
-    {
-        PyErr_Clear();
-    }
 #endif /* NCURSES_VERSION */
 
     SetDictInt("ERR", ERR);

Modules/_hashopenssl.c

@@ -118,15 +118,6 @@ _setException(PyObject *exc)
 }
 /* LCOV_EXCL_STOP */
 
-/* {Py_tp_new, NULL} doesn't block __new__ */
-static PyObject *
-_disabled_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
-{
-    PyErr_Format(PyExc_TypeError,
-        "cannot create '%.100s' instances", _PyType_Name(type));
-    return NULL;
-}
-
 static PyObject*
 py_digest_name(const EVP_MD *md)
 {
@@ -590,15 +581,14 @@ static PyType_Slot EVPtype_slots[] = {
     {Py_tp_doc, (char *)hashtype_doc},
     {Py_tp_methods, EVP_methods},
     {Py_tp_getset, EVP_getseters},
-    {Py_tp_new, _disabled_new},
     {0, 0},
 };
 
 static PyType_Spec EVPtype_spec = {
     "_hashlib.HASH",    /*tp_name*/
     sizeof(EVPobject),  /*tp_basicsize*/
     0,                  /*tp_itemsize*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_DISABLE_NEW,
     EVPtype_slots
 };
 
@@ -740,15 +730,14 @@ static PyType_Slot EVPXOFtype_slots[] = {
     {Py_tp_doc, (char *)hashxoftype_doc},
     {Py_tp_methods, EVPXOF_methods},
     {Py_tp_getset, EVPXOF_getseters},
-    {Py_tp_new, _disabled_new},
     {0, 0},
 };
 
 static PyType_Spec EVPXOFtype_spec = {
     "_hashlib.HASHXOF",    /*tp_name*/
     sizeof(EVPobject),  /*tp_basicsize*/
     0,                  /*tp_itemsize*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_DISABLE_NEW,
     EVPXOFtype_slots
 };
 
@@ -1734,14 +1723,13 @@ static PyType_Slot HMACtype_slots[] = {
     {Py_tp_dealloc,(destructor)_hmac_dealloc},
     {Py_tp_methods, HMAC_methods},
     {Py_tp_getset, HMAC_getset},
-    {Py_tp_new, _disabled_new},
     {0, NULL}
 };
 
 PyType_Spec HMACtype_spec = {
     "_hashlib.HMAC",    /* name */
     sizeof(HMACobject),     /* basicsize */
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
     .slots = HMACtype_slots,
 };
 

Modules/_tkinter.c

@@ -1002,7 +1002,7 @@ static PyType_Spec PyTclObject_Type_spec = {
     "_tkinter.Tcl_Obj",
     sizeof(PyTclObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
     PyTclObject_Type_slots,
 };
 
@@ -3294,7 +3294,7 @@ static PyType_Spec Tktt_Type_spec = {
     "_tkinter.tktimertoken",
     sizeof(TkttObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
     Tktt_Type_slots,
 };
 
@@ -3349,7 +3349,7 @@ static PyType_Spec Tkapp_Type_spec = {
     "_tkinter.tkapp",
     sizeof(TkappObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISABLE_NEW,
     Tkapp_Type_slots,
 };
 
@@ -3537,7 +3537,6 @@ PyInit__tkinter(void)
         Py_DECREF(m);
         return NULL;
     }
-    ((PyTypeObject *)o)->tp_new = NULL;
     if (PyModule_AddObject(m, "TkappType", o)) {
         Py_DECREF(o);
         Py_DECREF(m);
@@ -3550,7 +3549,6 @@ PyInit__tkinter(void)
         Py_DECREF(m);
         return NULL;
     }
-    ((PyTypeObject *)o)->tp_new = NULL;
     if (PyModule_AddObject(m, "TkttType", o)) {
         Py_DECREF(o);
         Py_DECREF(m);
@@ -3563,7 +3561,6 @@ PyInit__tkinter(void)
         Py_DECREF(m);
         return NULL;
     }
-    ((PyTypeObject *)o)->tp_new = NULL;
     if (PyModule_AddObject(m, "Tcl_Obj", o)) {
         Py_DECREF(o);
         Py_DECREF(m);

Objects/typeobject.c

@@ -1040,9 +1040,9 @@ type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
         }
     }
 
-    if (type->tp_new == NULL) {
+    if (type->tp_new == NULL || type->tp_flags & Py_TPFLAGS_DISABLE_NEW) {
         _PyErr_Format(tstate, PyExc_TypeError,
-                      "cannot create '%.100s' instances",
+                      "cannot create '%s' instances",
                       type->tp_name);
         return NULL;
     }
@@ -6824,8 +6824,15 @@ tp_new_wrapper(PyObject *self, PyObject *args, PyObject *kwds)
                      "__new__() called with non-type 'self'");
         return NULL;
     }
-
     type = (PyTypeObject *)self;
+
+    if (type->tp_flags & Py_TPFLAGS_DISABLE_NEW) {
+        PyErr_Format(PyExc_TypeError,
+                     "cannot create '%s' instances",
+                     type->tp_name);
+        return NULL;
+    }
+
     if (!PyTuple_Check(args) || PyTuple_GET_SIZE(args) < 1) {
         PyErr_Format(PyExc_TypeError,
                      "%s.__new__(): not enough arguments",

Python/sysmodule.c

@@ -2814,16 +2814,11 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
                                        &version_info_desc) < 0) {
             goto type_init_failed;
         }
+        /* prevent user from creating new instances */
+        VersionInfoType.tp_flags |= Py_TPFLAGS_DISABLE_NEW;
     }
     version_info = make_version_info(tstate);
     SET_SYS("version_info", version_info);
-    /* prevent user from creating new instances */
-    VersionInfoType.tp_init = NULL;
-    VersionInfoType.tp_new = NULL;
-    res = PyDict_DelItemString(VersionInfoType.tp_dict, "__new__");
-    if (res < 0 && _PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
-        _PyErr_Clear(tstate);
-    }
 
     /* implementation */
     SET_SYS("implementation", make_impl_info(version_info));
@@ -2833,33 +2828,20 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
         if (PyStructSequence_InitType2(&FlagsType, &flags_desc) < 0) {
             goto type_init_failed;
         }
+        /* prevent user from creating new instances */
+        FlagsType.tp_flags |= Py_TPFLAGS_DISABLE_NEW;
     }
     SET_SYS("flags", make_flags(tstate->interp));
-    /* prevent user from creating new instances */
-    FlagsType.tp_init = NULL;
-    FlagsType.tp_new = NULL;
-    res = PyDict_DelItemString(FlagsType.tp_dict, "__new__");
-    if (res < 0) {
-        if (!_PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
-            goto err_occurred;
-        }
-        _PyErr_Clear(tstate);
-    }
 
 #if defined(MS_WINDOWS)
     /* getwindowsversion */
-    if (WindowsVersionType.tp_name == 0)
+    if (WindowsVersionType.tp_name == 0) {
         if (PyStructSequence_InitType2(&WindowsVersionType,
                                        &windows_version_desc) < 0) {
             goto type_init_failed;
         }
-    /* prevent user from creating new instances */
-    WindowsVersionType.tp_init = NULL;
-    WindowsVersionType.tp_new = NULL;
-    assert(!_PyErr_Occurred(tstate));
-    res = PyDict_DelItemString(WindowsVersionType.tp_dict, "__new__");
-    if (res < 0 && _PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
-        _PyErr_Clear(tstate);
+        /* prevent user from creating new instances */
+        WindowsVersionType.tp_flags |= Py_TPFLAGS_DISABLE_NEW;
     }
 #endif