Fix #930: suppress CancelledError when Timeout raises TimeoutError

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

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__