Issue #27123: When an exception is raised within the context being

managed by a contextlib.ExitStack() and one of the exit stack
generators catches and raises it in a chain, do not re-raise the
original exception when exiting, let the new chained one through.
This avoids the PEP 479 bug described in issue25782.

Lib/test/test_contextlib.py

@@ -795,6 +795,34 @@ class TestExitStack(unittest.TestCase):
         stack.push(cm)
         self.assertIs(stack._exit_callbacks[-1], cm)
 
+    def test_dont_reraise_RuntimeError(self):
+        """https://bugs.python.org/issue27122"""
+        class UniqueException(Exception): pass
+
+        @contextmanager
+        def second():
+            try:
+                yield 1
+            except Exception as exc:
+                raise UniqueException("new exception") from exc
+
+        @contextmanager
+        def first():
+            try:
+                yield 1
+            except Exception as exc:
+                raise exc
+
+        # The RuntimeError should be caught by second()'s exception
+        # handler which chain raised a new UniqueException.
+        with self.assertRaises(UniqueException) as err_ctx:
+            with ExitStack() as es_ctx:
+                es_ctx.enter_context(second())
+                es_ctx.enter_context(first())
+                raise RuntimeError("please no infinite loop.")
+
+        self.assertEqual(err_ctx.exception.args[0], "new exception")
+
 
 class TestRedirectStream: