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, 9 insertions, 30 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 911c2d9..36297ff 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2810,56 +2810,35 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
 
     /* version_info */
     if (VersionInfoType.tp_name == NULL) {
-        if (PyStructSequence_InitType2(&VersionInfoType,
-                                       &version_info_desc) < 0) {
+        if (_PyStructSequence_InitType(&VersionInfoType,
+                                       &version_info_desc,
+                                       Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
             goto type_init_failed;
         }
     }
     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));
 
     // sys.flags: updated in-place later by _PySys_UpdateConfig()
     if (FlagsType.tp_name == 0) {
-        if (PyStructSequence_InitType2(&FlagsType, &flags_desc) < 0) {
+        if (_PyStructSequence_InitType(&FlagsType, &flags_desc,
+                                       Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
             goto type_init_failed;
         }
     }
     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 (PyStructSequence_InitType2(&WindowsVersionType,
-                                       &windows_version_desc) < 0) {
+    if (WindowsVersionType.tp_name == 0) {
+        if (_PyStructSequence_InitType(&WindowsVersionType,
+                                       &windows_version_desc,
+                                       Py_TPFLAGS_DISALLOW_INSTANTIATION) < 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);
     }
 #endif