bpo-36060: Document how collections.ChainMap() determines iteration order (GH-11969)

Lib/test/test_collections.py

@@ -113,6 +113,20 @@ class TestChainMap(unittest.TestCase):
         self.assertEqual(f['b'], 5)                                    # find first in chain
         self.assertEqual(f.parents['b'], 2)                            # look beyond maps[0]
 
+    def test_ordering(self):
+        # Combined order matches a series of dict updates from last to first.
+        # This test relies on the ordering of the underlying dicts.
+
+        baseline = {'music': 'bach', 'art': 'rembrandt'}
+        adjustments = {'art': 'van gogh', 'opera': 'carmen'}
+
+        cm = ChainMap(adjustments, baseline)
+
+        combined = baseline.copy()
+        combined.update(adjustments)
+
+        self.assertEqual(list(combined.items()), list(cm.items()))
+
     def test_constructor(self):
         self.assertEqual(ChainMap().maps, [{}])                        # no-args --> one new dict
         self.assertEqual(ChainMap({1:2}).maps, [{1:2}])                # 1 arg --> list