bpo-34427: Fix infinite loop when calling MutableSequence.extend() on self (GH-8813)

Lib/_collections_abc.py

@@ -986,6 +986,8 @@ class MutableSequence(Sequence):
 
     def extend(self, values):
         'S.extend(iterable) -- extend sequence by appending elements from the iterable'
+        if values is self:
+            values = list(values)
         for v in values:
             self.append(v)
 

Lib/test/test_collections.py

@@ -1721,6 +1721,18 @@ class TestCollectionABCs(ABCTestCase):
         mss.clear()
         self.assertEqual(len(mss), 0)
 
+        # issue 34427
+        # extending self should not cause infinite loop
+        items = 'ABCD'
+        mss2 = MutableSequenceSubclass()
+        mss2.extend(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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Misc/NEWS.d/next/Library/2018-08-20-13-53-10.bpo-34427.tMRQjl.rst

+Fix infinite loop in ``a.extend(a)`` for ``MutableSequence`` subclasses.