Improve error messages for bounded iteration on extended slice assignment

Co-authored-by: csm10495 <5749838+csm10495@users.noreply.github.com>
Agent-Logs-Url: https://github.com/csm10495/cpython/sessions/48a4582d-926d-44bf-aa9b-8078648099f4

Author: Copilot

Lib/test/list_tests.py

@@ -210,8 +210,10 @@ def infinite_gen():
                 yield "foo"
 
         a = self.type2test(range(4))
-        with self.assertRaises(ValueError):
+        with self.assertRaises(ValueError) as cm:
             a[::2] = infinite_gen()
+        self.assertIn("yielded more items than extended slice of size 2",
+                       str(cm.exception))
         # list should be unchanged after the failed assignment
         self.assertEqual(a, self.type2test(range(4)))
 
@@ -235,14 +237,18 @@ def test_extended_slice_assign_iterator(self):
 
         # Too few items from an iterator
         a = self.type2test(range(10))
-        with self.assertRaises(ValueError):
+        with self.assertRaises(ValueError) as cm:
             a[::2] = iter(range(3))
+        self.assertIn("sequence of size 3 to extended slice of larger size",
+                       str(cm.exception))
         self.assertEqual(a, self.type2test(range(10)))
 
         # Too many items from an iterator
         a = self.type2test(range(10))
-        with self.assertRaises(ValueError):
+        with self.assertRaises(ValueError) as cm:
             a[::2] = iter(range(10))
+        self.assertIn("yielded more items than extended slice of size 5",
+                       str(cm.exception))
         self.assertEqual(a, self.type2test(range(10)))
 
     def test_delslice(self):

Objects/listobject.c

@@ -3821,6 +3821,7 @@ list_ass_subscript_lock_held(PyObject *_self, PyObject *item, PyObject *value)
             PyObject **garbage, **seqitems, **selfitems;
             Py_ssize_t i;
             size_t cur;
+            int bounded_iter = 0;
 
             /* protect against a[::-1] = a */
             if (self == (PyListObject*)value) {
@@ -3882,6 +3883,7 @@ list_ass_subscript_lock_held(PyObject *_self, PyObject *item, PyObject *value)
                 if (j < alloc) {
                     Py_SET_SIZE(seq, j);
                 }
+                bounded_iter = 1;
             }
             else {
                 seq = PySequence_Fast(value,
@@ -3901,12 +3903,29 @@ list_ass_subscript_lock_held(PyObject *_self, PyObject *item, PyObject *value)
             }
 
             if (PySequence_Fast_GET_SIZE(seq) != slicelength) {
-                PyErr_Format(PyExc_ValueError,
-                    "attempt to assign sequence of "
-                    "size %zd to extended slice of "
-                    "size %zd",
-                         PySequence_Fast_GET_SIZE(seq),
-                         slicelength);
+                if (bounded_iter &&
+                    PySequence_Fast_GET_SIZE(seq) > slicelength) {
+                    PyErr_Format(PyExc_ValueError,
+                        "attempt to assign iterable that yielded "
+                        "more items than extended slice of "
+                        "size %zd",
+                             slicelength);
+                }
+                else if (bounded_iter) {
+                    PyErr_Format(PyExc_ValueError,
+                        "attempt to assign sequence of "
+                        "size %zd to extended slice of "
+                        "larger size",
+                             PySequence_Fast_GET_SIZE(seq));
+                }
+                else {
+                    PyErr_Format(PyExc_ValueError,
+                        "attempt to assign sequence of "
+                        "size %zd to extended slice of "
+                        "size %zd",
+                             PySequence_Fast_GET_SIZE(seq),
+                             slicelength);
+                }
                 Py_DECREF(seq);
                 return -1;
             }