Python asyncio: how to mock __aiter__() method?

I have a code which is listening to messages on WebSocket using aiohttp.

It looks like:

async for msg in ws:
    await self._ws_msg_handler.handle_message(ws, msg, _services)

Where ws is an instance of aiohttp.web.WebSocketResponse() (original code)

In my test I mock WebSocketResponse() and its __aiter__ method:

def coro_mock(**kwargs):
    return asyncio.coroutine(mock.Mock(**kwargs))


@pytest.mark.asyncio
@mock.patch('aiojsonrpc.request_handler.WebSocketMessageHandler')
async def test_rpc_websocket_handler(
    MockWebSocketMessageHandler,
    rpc_websocket_handler
):

    ws_response = 'aiojsonrpc.request_handler.WebSocketResponse'
    with mock.patch(ws_response) as MockWebSocketResponse:
        MockRequest = mock.MagicMock()
        req = MockRequest()

        ws_instance = MockWebSocketResponse.return_value
        ws_instance.prepare = coro_mock()
        ws_instance.__aiter__ = coro_mock(return_value=iter(range(5)))
        ws_instance.__anext__ = coro_mock()

        handle_msg_result = 'Message processed'
        MockWebSocketMessageHandler.handle_message.side_effect = Exception(
            handle_msg_result)
        msg_handler = MockWebSocketMessageHandler()

        with pytest.raises(Exception) as e:
            await request_handler.RpcWebsocketHandler(msg_handler)(req)
        assert str(e.value) == handle_msg_result

Though when I run the test it fails with the error message saying:

'async for' requires an object with __aiter__ method, got MagicMock

=================================================================================== FAILURES ===================================================================================
__________________________________________________________________________ test_rpc_websocket_handler __________________________________________________________________________

MockWebSocketMessageHandler = <MagicMock name='WebSocketMessageHandler' id='140687969989632'>
rpc_websocket_handler = <aiojsonrpc.request_handler.RpcWebsocketHandler object at 0x7ff47879b0f0>

    @pytest.mark.asyncio
    @mock.patch('aiojsonrpc.request_handler.WebSocketMessageHandler')
    async def test_rpc_websocket_handler(
        MockWebSocketMessageHandler,
        rpc_websocket_handler
    ):

        ws_response = 'aiojsonrpc.request_handler.WebSocketResponse'
        with mock.patch(ws_response) as MockWebSocketResponse:
            # MockRequest = mock.create_autospec(aiohttp.web_reqrep.Request)
            # req = MockRequest(*[None] * 6)
            MockRequest = mock.MagicMock()
            req = MockRequest()

            ws_instance = MockWebSocketResponse.return_value
            ret = mock.Mock()
            ws_instance.prepare = coro_mock()
            ws_instance.__aiter__ = coro_mock(return_value=iter(range(5)))
            ws_instance.__anext__ = coro_mock()

            handle_msg_result = 'Message processed'
            MockWebSocketMessageHandler.handle_message.side_effect = Exception(
                handle_msg_result)
            msg_handler = MockWebSocketMessageHandler()

            with pytest.raises(Exception) as e:
                await request_handler.RpcWebsocketHandler(msg_handler)(req)
>           assert str(e.value) == handle_msg_result
E           assert "'async for' ...got MagicMock" == 'Message processed'
E             - 'async for' requires an object with __aiter__ method, got MagicMock
E             + Message processed

tests/test_request_handler.py:252: AssertionError

So it behaves like __aiter__() was never mocked. How I'm supposed to accomplish correct mocking in this case?

---

Update:

For now I've found a workaround to make the code testable though I would really appreciate if someone tell me how to deal with the issue described in the original question.

You can make the mocked class return an object implementing the expected interface:

class AsyncIterator:
    def __init__(self, seq):
        self.iter = iter(seq)

    def __aiter__(self):
        return self

    async def __anext__(self):
        try:
            return next(self.iter)
        except StopIteration:
            raise StopAsyncIteration

MockWebSocketResponse.return_value = AsyncIterator(range(5))

I don't think there is a way (yet) to correctly mock an object implementing __aiter__, it may be a python bug, as async for rejects a MagicMock, even if hasattr(the_magic_mock, '__aiter__') is True.

EDIT (13/12/2017): the library asynctest supports asynchronous iterators and context managers since 0.11, asynctest.MagicMock provides this feature for free.