bpo-26915: Test identity first in membership operation in index() and count() methods of collections.abc.Sequence (GH-503)

Lib/test/test_collections.py

@@ -1309,20 +1309,29 @@ class TestCollectionABCs(ABCTestCase):
         class CustomEqualObject:
             def __eq__(self, other):
                 return False
-        class CustomSequence(list):
-            def __contains__(self, value):
-                return Sequence.__contains__(self, value)
+        class CustomSequence(Sequence):
+            def __init__(self, seq):
+                self._seq = seq
+            def __getitem__(self, index):
+                return self._seq[index]
+            def __len__(self):
+                return len(self._seq)
 
         nan = float('nan')
         obj = CustomEqualObject()
+        seq = CustomSequence([nan, obj, nan])
         containers = [
-            CustomSequence([nan, obj]),
+            seq,
             ItemsView({1: nan, 2: obj}),
             ValuesView({1: nan, 2: obj})
         ]
         for container in containers:
             for elem in container:
                 self.assertIn(elem, container)
+        self.assertEqual(seq.index(nan), 0)
+        self.assertEqual(seq.index(obj), 1)
+        self.assertEqual(seq.count(nan), 2)
+        self.assertEqual(seq.count(obj), 1)
 
     def assertSameSet(self, s1, s2):
         # coerce both to a real set then check equality