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

3 files changed, 15 insertions, 0 deletions

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index dbe30dff1f..c363987970 100644
--- a/Lib/_collections_abc.py
+++ b/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)
 
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 2099d236d0..0b7cb5848b 100644
--- a/Lib/test/test_collections.py
+++ b/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))
+
+
 ################################################################################
 ### Counter
 ################################################################################
diff --git a/Misc/NEWS.d/next/Library/2018-08-20-13-53-10.bpo-34427.tMRQjl.rst b/Misc/NEWS.d/next/Library/2018-08-20-13-53-10.bpo-34427.tMRQjl.rst
new file mode 100644
index 0000000000..f6e0e030b7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-08-20-13-53-10.bpo-34427.tMRQjl.rst
@@ -0,0 +1 @@
+Fix infinite loop in ``a.extend(a)`` for ``MutableSequence`` subclasses.