Issue #26492: Added additional tests for exhausted iterators of mutable sequences.

Lib/test/list_tests.py

@@ -593,3 +593,14 @@ class F(object):
             def __iter__(self):
                 raise KeyboardInterrupt
         self.assertRaises(KeyboardInterrupt, list, F())
+
+    def test_exhausted_iterator(self):
+        a = self.type2test([1, 2, 3])
+        exhit = iter(a)
+        empit = iter(a)
+        for x in exhit:  # exhaust the iterator
+            next(empit)  # not exhausted
+        a.append(9)
+        self.assertEqual(list(exhit), [])
+        self.assertEqual(list(empit), [9])
+        self.assertEqual(a, self.type2test([1, 2, 3, 9]))

Lib/test/test_bytes.py

@@ -17,6 +17,7 @@
 import test.support
 import test.string_tests
 import test.buffer_tests
+import test.list_tests
 from test.support import bigaddrspacetest, MAX_Py_ssize_t
 
 
@@ -1418,6 +1419,7 @@ def test_iterator_pickling2(self):
             b[:] = data
             self.assertEqual(list(it), [])
 
+    test_exhausted_iterator = test.list_tests.CommonTest.test_exhausted_iterator
 
 class AssortedBytesTest(unittest.TestCase):
     #

Lib/test/test_iter.py

@@ -190,6 +190,17 @@ def test_mutating_seq_class_iter_pickle(self):
             self.assertTrue(isinstance(it, collections.abc.Iterator))
             self.assertEqual(list(it), [])
 
+    def test_mutating_seq_class_exhausted_iter(self):
+        a = SequenceClass(5)
+        exhit = iter(a)
+        empit = iter(a)
+        for x in exhit:  # exhaust the iterator
+            next(empit)  # not exhausted
+        a.n = 7
+        self.assertEqual(list(exhit), [])
+        self.assertEqual(list(empit), [5, 6])
+        self.assertEqual(list(a), [0, 1, 2, 3, 4, 5, 6])
+
     # Test a new_style class with __iter__ but no next() method
     def test_new_style_iter_class(self):
         class IterClass(object):