bpo-29692: contextlib.contextmanager may incorrectly unchain RuntimeError (GH-949)

contextlib._GeneratorContextManager.__exit__ includes a special case to deal with
PEP 479 RuntimeErrors created when `StopIteration` is thrown into the context
manager body.

Previously this check was too permissive, and undid one level of chaining on *all*
RuntimeError instances, not just those that wrapped a StopIteration instance.

1 files changed, 23 insertions, 0 deletions

diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index c04c804af5..b1a467d952 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -152,6 +152,29 @@ def woohoo():
         else:
             self.fail('StopIteration was suppressed')
 
+    def test_contextmanager_do_not_unchain_non_stopiteration_exceptions(self):
+        @contextmanager
+        def test_issue29692():
+            try:
+                yield
+            except Exception as exc:
+                raise RuntimeError('issue29692:Chained') from exc
+        try:
+            with test_issue29692():
+                raise ZeroDivisionError
+        except Exception as ex:
+            self.assertIs(type(ex), RuntimeError)
+            self.assertEqual(ex.args[0], 'issue29692:Chained')
+            self.assertIsInstance(ex.__cause__, ZeroDivisionError)
+
+        try:
+            with test_issue29692():
+                raise StopIteration('issue29692:Unchained')
+        except Exception as ex:
+            self.assertIs(type(ex), StopIteration)
+            self.assertEqual(ex.args[0], 'issue29692:Unchained')
+            self.assertIsNone(ex.__cause__)
+
     def _create_contextmanager_attribs(self):
         def attribs(**kw):
             def decorate(func):