aiohttp unittest: Client Session Request Requires Absolute URL

Question

I am trying to write a test for a method that uses aiohttp's ClientSession. I am using Python's unittest framework.

I followed the approach described here: https://docs.aiohttp.org/en/stable/testing.html#unittest:

from aiohttp.test_utils import AioHTTPTestCase
from aiohttp import web

class MyAppTestCase(AioHTTPTestCase):

    async def get_application(self):
        """
        Override the get_app method to return your application.
        """
        async def hello(request):
            return web.Response(text='Hello, world')

        app = web.Application()
        app.router.add_get('/', hello) # relative URL
        return app

    async def test_example(self):
        async with self.client.request("GET", "/") as resp:
            self.assertEqual(resp.status, 200)
            text = await resp.text()
        self.assertIn("Hello, world", text)

Unlike In the given example, I would like to use ClientSession:

async with self.client.session as sess:
    resp = await sess.get('/') # here, I'd like to use an absolute URL

ClientSession requires an absolute URL, see https://github.com/aio-libs/aiohttp/blob/ed196893b6102cedcc98251c0c76d1b9c5103cee/aiohttp/test_utils.py#L298-L302.

I looked at aiohttp unittest with URLs not GET but I do not understand how the approach with the responses lib would fit into my setup. I got

TypeError: The first argument should be web.Application instance, got None