bpo-43916: Add Py_TPFLAGS_DISALLOW_INSTANTIATION type flag (GH-25721)

Add a new Py_TPFLAGS_DISALLOW_INSTANTIATION type flag to disallow
creating type instances: set tp_new to NULL and don't create the
"__new__" key in the type dictionary.

The flag is set automatically on static types if tp_base is NULL or
&PyBaseObject_Type and tp_new is NULL.

Use the flag on the following types:

* _curses.ncurses_version type
* _curses_panel.panel
* _tkinter.Tcl_Obj
* _tkinter.tkapp
* _tkinter.tktimertoken
* _xxsubinterpretersmodule.ChannelID
* sys.flags type
* sys.getwindowsversion() type
* sys.version_info type

Update MyStr example in the C API documentation to use
Py_TPFLAGS_DISALLOW_INSTANTIATION.

Add _PyStructSequence_InitType() function to create a structseq type
with the Py_TPFLAGS_DISALLOW_INSTANTIATION flag set.

type_new() calls _PyType_CheckConsistency() at exit.

4 files changed, 14 insertions, 33 deletions

diff --git a/Modules/_curses_panel.c b/Modules/_curses_panel.c
index 94caf8c..0b328f9 100644
--- a/Modules/_curses_panel.c
+++ b/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_DISALLOW_INSTANTIATION,
     .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;
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index d221cf1..bcf9ad1 100644
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -4793,9 +4793,11 @@ PyInit__curses(void)
 #ifdef NCURSES_VERSION
     /* ncurses_version */
     if (NcursesVersionType.tp_name == NULL) {
-        if (PyStructSequence_InitType2(&NcursesVersionType,
-                                       &ncurses_version_desc) < 0)
+        if (_PyStructSequence_InitType(&NcursesVersionType,
+                                       &ncurses_version_desc,
+                                       Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
             return NULL;
+        }
     }
     v = make_ncurses_version();
     if (v == NULL) {
@@ -4803,15 +4805,6 @@ PyInit__curses(void)
     }
     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);
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 46d6a6e..3a0e5de 100644
--- a/Modules/_tkinter.c
+++ b/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_DISALLOW_INSTANTIATION,
     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_DISALLOW_INSTANTIATION,
     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_DISALLOW_INSTANTIATION,
     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);
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index b94b130..9290255 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1780,7 +1780,12 @@ static PyTypeObject ChannelIDtype = {
     0,                              /* tp_getattro */
     0,                              /* tp_setattro */
     0,                              /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
+    // Use Py_TPFLAGS_DISALLOW_INSTANTIATION so the type cannot be instantiated
+    // from Python code.  We do this because there is a strong relationship
+    // between channel IDs and the channel lifecycle, so this limitation avoids
+    // related complications. Use the _channel_id() function instead.
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
+        | Py_TPFLAGS_DISALLOW_INSTANTIATION, /* tp_flags */
     channelid_doc,                  /* tp_doc */
     0,                              /* tp_traverse */
     0,                              /* tp_clear */
@@ -1791,19 +1796,6 @@ static PyTypeObject ChannelIDtype = {
     0,                              /* tp_methods */
     0,                              /* tp_members */
     channelid_getsets,              /* tp_getset */
-    0,                              /* tp_base */
-    0,                              /* tp_dict */
-    0,                              /* tp_descr_get */
-    0,                              /* tp_descr_set */
-    0,                              /* tp_dictoffset */
-    0,                              /* tp_init */
-    0,                              /* tp_alloc */
-    // Note that we do not set tp_new to channelid_new.  Instead we
-    // set it to NULL, meaning it cannot be instantiated from Python
-    // code.  We do this because there is a strong relationship between
-    // channel IDs and the channel lifecycle, so this limitation avoids
-    // related complications.
-    NULL,                           /* tp_new */
 };