Testing aiohttp client with unittest.mock.patch

I've written a simple HTTP client using aiohttp and I'm trying to test it by patching aiohttp.ClientSession and aiohttp.ClientResponse. However, it appears as though the unittest.mock.patch decorator is not respecting my asynchronous code. At a guess, I would say it's some kind of namespacing mismatch.

Here's a minimal example:

from aiohttp import ClientSession

async def is_ok(url:str) -> bool:
    async with ClientSession() as session:
        async with session.request("GET", url) as response:
            return (response.status == 200)

I'm using an asynchronous decorator for testing, as described in this answer. So here's my attempted test:

import unittest
from unittest.mock import MagicMock, patch

from aiohttp import ClientResponse

from my.original.module import is_ok

class TestClient(unittest.TestCase):
    @async_test
    @patch("my.original.module.ClientSession", spec=True)
    async def test_client(self, mock_client):
        mock_response = MagicMock(spec=ClientResponse)
        mock_response.status = 200

        async def _mock_request(*args, **kwargs):
            return mock_response

        mock_client.request = mock_response

        status = await is_ok("foo")
        self.assertTrue(status)

My is_ok coroutine works fine when it's used in, say, __main__, but when I run the test, it gives me an error that indicates that the session.request function has not been mocked per my patch call. (Specifically it says "Could not parse hostname from URL 'foo'", which it should if it weren't mocked.)

I am unable to escape this behaviour. I have tried:

• Importing is_ok after the mocking is done.
• Various combinations of assigning mocks to mock_client and mock_client.__aenter__, setting mock_client.request to MagicMock(return_value=mock_response), or using mock_client().request, etc.
• Writing a mock ClientSession with specific __aenter__ and __aexit__ methods and using it in the new argument to patch.

None of these appear to make a difference. If I put assertions into is_ok to test that ClientSession is an instance of MagicMock, then these assertions fail when I run the test (as, again, they would when the code is not patched). That leads me to my namespacing mismatch theory: That is, the event loop is running in a different namespace to which patch is targeting.

Either that, or I'm doing something stupid!