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.

1 files changed, 4 insertions, 11 deletions

diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index d221cf1a92..bcf9ad1938 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);