gh-141367: Use CALL_LIST_APPEND instruction only for lists, not for list subclasses

Lib/test/test_opcache.py

@@ -1872,6 +1872,33 @@ def for_iter_generator():
         self.assert_specialized(for_iter_generator, "FOR_ITER_GEN")
         self.assert_no_opcode(for_iter_generator, "FOR_ITER")
 
+    @cpython_only
+    @requires_specialization_ft
+    def test_call_list_append(self):
+        # gh-141367: only exact lists should use
+        # CALL_LIST_APPEND instruction after specialization.
+
+        r = range(_testinternalcapi.SPECIALIZATION_THRESHOLD)
+
+        def list_append(l):
+            for _ in r:
+                l.append(1)
+
+        list_append([])
+        self.assert_specialized(list_append, "CALL_LIST_APPEND")
+        self.assert_no_opcode(list_append, "CALL_METHOD_DESCRIPTOR_O")
+        self.assert_no_opcode(list_append, "CALL")
+
+        def my_list_append(l):
+            for _ in r:
+                l.append(1)
+
+        class MyList(list): pass
+        my_list_append(MyList())
+        self.assert_specialized(my_list_append, "CALL_METHOD_DESCRIPTOR_O")
+        self.assert_no_opcode(my_list_append, "CALL_LIST_APPEND")
+        self.assert_no_opcode(my_list_append, "CALL")
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-11-11-13-40-45.gh-issue-141367.I5KY7F.rst

@@ -0,0 +1,2 @@
+Use ``CALL_LIST_APPEND`` instruction only for lists, not for list
+subclasses, to avoid unnecessary deopt. Patch by Mikhail Efimov.

Python/specialize.c

@@ -19,6 +19,7 @@
 #include "pycore_pylifecycle.h"   // _PyOS_URandomNonblock()
 #include "pycore_runtime.h"       // _Py_ID()
 #include "pycore_unicodeobject.h" // _PyUnicodeASCIIIter_Type
+#include "pycore_pystate.h"       // _PyThreadState_GET()
 
 #include <stdlib.h> // rand()
 
@@ -1627,8 +1628,13 @@ specialize_method_descriptor(PyMethodDescrObject *descr, _Py_CODEUNIT *instr,
             bool pop = (next.op.code == POP_TOP);
             int oparg = instr->op.arg;
             if ((PyObject *)descr == list_append && oparg == 1 && pop) {
-                specialize(instr, CALL_LIST_APPEND);
-                return 0;
+                PyThreadState *tstate = _PyThreadState_GET();
+                _PyStackRef *stack_pointer = tstate->current_frame->stackpointer;
+                PyObject *self = PyStackRef_AsPyObjectBorrow(stack_pointer[-2]);
+                if (PyList_CheckExact(self)) {
+                    specialize(instr, CALL_LIST_APPEND);
+                    return 0;
+                }
             }
             specialize(instr, CALL_METHOD_DESCRIPTOR_O);
             return 0;