From e92e817e888e239d08cde1ca757ab4c250b13351 Mon Sep 17 00:00:00 2001
From: Andrew Svetlov <andrew.svetlov@gmail.com>
Date: Sat, 16 Jul 2016 20:44:45 +0300
Subject: [PATCH] Fix #930: suppress CancelledError when Timeout raises
 TimeoutError

---
 aiohttp/helpers.py    |  2 +-
 tests/test_timeout.py | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/aiohttp/helpers.py b/aiohttp/helpers.py
index 256a55882b0..d93d6c5bb21 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -561,7 +561,7 @@ def __exit__(self, exc_type, exc_val, exc_tb):
         if exc_type is asyncio.CancelledError and self._cancelled:
             self._cancel_handler = None
             self._task = None
-            raise asyncio.TimeoutError
+            raise asyncio.TimeoutError from None
         if self._timeout is not None:
             self._cancel_handler.cancel()
             self._cancel_handler = None
diff --git a/tests/test_timeout.py b/tests/test_timeout.py
index 7d34a24cbf6..07e13a5c5ae 100644
--- a/tests/test_timeout.py
+++ b/tests/test_timeout.py
@@ -165,3 +165,13 @@ def outer():
         yield from task
     assert task.cancelled()
     assert task.done()
+
+
+@pytest.mark.run_loop
+def test_timeout_suppress_exception_chain(loop):
+
+    with pytest.raises(asyncio.TimeoutError) as ctx:
+        with Timeout(0.01, loop=loop) as t:
+            yield from asyncio.sleep(10, loop=loop)
+            assert t._loop is loop
+    assert ctx.value.__suppress_context__