added tests to mutable sequence subclass

Author: KittenHero

Lib/test/test_collections.py

@@ -1721,6 +1721,17 @@ def insert(self, index, value):
         mss.clear()
         self.assertEqual(len(mss), 0)
 
+        # issue 34427
+        # extending self should not cause infinite loop
+        items = 'ABCD'
+        mss2 = MutableSequenceSubclass(items + items)
+        mss.clear()
+        mss.extend(items)
+        mss.extend(mss)
+        self.assertEqual(len(mss), len(mss2))
+        self.assertEqual(list(mss), list(mss2))
+
+
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