gh-125038: Raise TypeError on such case instead of silently stopping the loop

Author: efimov-mikhail

Lib/test/test_generators.py

@@ -269,10 +269,22 @@ def loop():
         loop()
 
     def test_issue125038(self):
-        g = (x for x in range(10))
-        g.gi_frame.f_locals['.0'] = range(20)
-        l = list(g)
-        self.assertListEqual(l, [])
+        def get_generator():
+            g = (x for x in range(10))
+            g.gi_frame.f_locals['.0'] = range(20)
+            return g
+
+        def genexpr_to_list():
+            try:
+                l = list(get_generator())
+                return "NoError"
+            except TypeError:
+                return "TypeError"
+
+        # This should not raise
+        r = genexpr_to_list()
+
+        self.assertIs(r, "TypeError")
 
 class ExceptionTest(unittest.TestCase):
     # Tests for the issue #23353: check that the currently handled exception

Python/bytecodes.c

@@ -2804,10 +2804,16 @@ dummy_func(
         replaced op(_FOR_ITER, (iter -- iter, next)) {
             /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
-            PyObject *next_o = NULL;
-            if (PyIter_Check(iter_o)) {
-                next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+            PyTypeObject *type = Py_TYPE(iter_o);
+            iternextfunc iternext = type->tp_iternext;
+            if (iternext == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'for' requires an object with "
+                              "__iter__ method, got %.100s",
+                              type->tp_name);
+                ERROR_NO_POP();
             }
+            PyObject *next_o = (*iternext)(iter_o);
             if (next_o == NULL) {
                 if (_PyErr_Occurred(tstate)) {
                     int matches = _PyErr_ExceptionMatches(tstate, PyExc_StopIteration);
@@ -2833,10 +2839,16 @@ dummy_func(
         op(_FOR_ITER_TIER_TWO, (iter -- iter, next)) {
             /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
-            PyObject *next_o = NULL;
-            if (PyIter_Check(iter_o)) {
-                next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+            PyTypeObject *type = Py_TYPE(iter_o);
+            iternextfunc iternext = type->tp_iternext;
+            if (iternext == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'for' requires an object with "
+                              "__iter__ method, got %.100s",
+                              type->tp_name);
+                ERROR_NO_POP();
             }
+            PyObject *next_o = (*iternext)(iter_o);
             if (next_o == NULL) {
                 if (_PyErr_Occurred(tstate)) {
                     int matches = _PyErr_ExceptionMatches(tstate, PyExc_StopIteration);
@@ -2860,10 +2872,16 @@ dummy_func(
             _Py_CODEUNIT *target;
             _PyStackRef iter_stackref = TOP();
             PyObject *iter = PyStackRef_AsPyObjectBorrow(iter_stackref);
-            PyObject *next = NULL;
-            if (PyIter_Check(iter)) {
-                next = (*Py_TYPE(iter)->tp_iternext)(iter);
+            PyTypeObject *type = Py_TYPE(iter);
+            iternextfunc iternext = type->tp_iternext;
+            if (iternext == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'for' requires an object with "
+                              "__iter__ method, got %.100s",
+                              type->tp_name);
+                ERROR_NO_POP();
             }
+            PyObject *next = (*iternext)(iter);
             if (next != NULL) {
                 PUSH(PyStackRef_FromPyObjectSteal(next));
                 target = next_instr;