bpo-29942: Fix the use of recursion in itertools.chain.from_iterable. (#911)

* bpo-29942: Fix the use of recursion in itertools.chain.from_iterable.

Fix the use of recursion in itertools.chain.from_iterable. Using recursion
is unnecessary, and can easily cause stack overflows, especially when
building in low optimization modes or with Py_DEBUG enabled.
(cherry picked from commit 5466d4af5fe76ec0a5fbc8a05675287d9e8e9d14)

1 files changed, 8 insertions, 0 deletions

diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index ea1f57c..c431f0d 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -1976,6 +1976,14 @@ class RegressionTests(unittest.TestCase):
         self.assertRaises(AssertionError, list, cycle(gen1()))
         self.assertEqual(hist, [0,1])
 
+    def test_long_chain_of_empty_iterables(self):
+        # Make sure itertools.chain doesn't run into recursion limits when
+        # dealing with long chains of empty iterables. Even with a high
+        # number this would probably only fail in Py_DEBUG mode.
+        it = chain.from_iterable(() for unused in range(10000000))
+        with self.assertRaises(StopIteration):
+            next(it)
+
 class SubclassWithKwargsTest(unittest.TestCase):
     def test_keywords_in_subclass(self):
         # count is not subclassable...