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, 3 insertions, 6 deletions

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);