gh-149180: Avoid double checking tp_as_number, tp_as_sequence, tp_as_mapping

Include/internal/pycore_abstract.h

@@ -13,7 +13,7 @@ static inline int
 _PyIndex_Check(PyObject *obj)
 {
     PyNumberMethods *tp_as_number = Py_TYPE(obj)->tp_as_number;
-    return (tp_as_number != NULL && tp_as_number->nb_index != NULL);
+    return (tp_as_number->nb_index != NULL);
 }
 
 // Exported for external JIT support

Misc/NEWS.d/next/Core_and_Builtins/2026-05-04-10-35-49.gh-issue-149180.wCERVE.rst

@@ -0,0 +1,5 @@
+Ensure :c:member:`~PyTypeObject.tp_as_number`,
+:c:member:`~PyTypeObject.tp_as_sequence`, and
+:c:member:`~PyTypeObject.tp_as_mapping` are never ``NULL`` after
+:c:func:`PyType_Ready` by assigning shared empty structs as fallbacks.
+Remove redundant ``NULL`` checks at callsites.

Modules/_bisectmodule.c

@@ -35,11 +35,11 @@ get_sq_item(PyObject *s)
     // The parts of PySequence_GetItem that we only need to do once
     PyTypeObject *tp = Py_TYPE(s);
     PySequenceMethods *m = tp->tp_as_sequence;
-    if (m && m->sq_item) {
+    if (m->sq_item) {
         return m->sq_item;
     }
     const char *msg;
-    if (tp->tp_as_mapping && tp->tp_as_mapping->mp_subscript) {
+    if (tp->tp_as_mapping->mp_subscript) {
         msg = "%.200s is not a sequence";
     }
     else {