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

[Raymond Hettinger]

https://github.com/python/cpython/commit/1b5f9c9653f348b0aa8b7ca39f8a9361150f7dfc
commit: 1b5f9c9653f348b0aa8b7ca39f8a9361150f7dfc
branch: master
author: Naris R <nariscatboy at gmail.com>
committer: Raymond Hettinger <rhettinger at users.noreply.github.com>
date: 2018-08-30T09:56:14-07:00
summary:

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

files:
A Misc/NEWS.d/next/Library/2018-08-20-13-53-10.bpo-34427.tMRQjl.rst
M Lib/_collections_abc.py
M Lib/test/test_collections.py

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index dbe30dff1fe1..c363987970b4 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -986,6 +986,8 @@ def reverse(self):
 
     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 2099d236d0c4..0b7cb5848b1b 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -1721,6 +1721,18 @@ 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()
+        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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 ### Counter
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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 000000000000..f6e0e030b7fe
--- /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.