diff -r 9649acf7d472 Lib/contextlib.py
--- a/Lib/contextlib.py	Sat Jun 11 18:02:46 2016 -0700
+++ b/Lib/contextlib.py	Sat Jun 11 18:58:22 2016 -0700
@@ -82,6 +82,9 @@
                 # raised inside the "with" statement from being suppressed.
                 return exc is not value
             except RuntimeError as exc:
+                # Don't re-raise the passed in exception. (issue27112)
+                if exc is value:
+                    return False
                 # Likewise, avoid suppressing if a StopIteration exception
                 # was passed to throw() and later wrapped into a RuntimeError
                 # (see PEP 479).
diff -r 9649acf7d472 Lib/test/test_contextlib.py
--- a/Lib/test/test_contextlib.py	Sat Jun 11 18:02:46 2016 -0700
+++ b/Lib/test/test_contextlib.py	Sat Jun 11 18:58:22 2016 -0700
@@ -762,6 +762,34 @@
         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: