AssertionError “assert not IOLoop.initialized()” o

I have some trouble with writing tests with AsyncHTTPTestCase for existing Tornado application that uses asyncio event loop. Here I prepare short model where I can reproduce issue:

app.py

from tornado.platform.asyncio import AsyncIOMainLoop
import asyncio
import tornado.web


class MainHandler(tornado.web.RequestHandler):
    async def get(self, *args, **kwargs):
        self.write("200 OK")

    async def post(self, *args, **kwargs):
        self.write("201 OK")

def make_app():
    AsyncIOMainLoop().install()  # here is how to asyncio loop installed in app I already have
    return tornado.web.Application([
        (r"/", MainHandler),
    ], debug=True)

def start_app():
    app = make_app()
    app.listen(8888)
    loop = asyncio.get_event_loop()
    loop.set_debug(True)
    loop.run_forever()

start.py

#!/usr/bin/env python3
import app

if __name__ == "__main__":
    app.start_app()

test_app.py

import json
from tornado.testing import AsyncHTTPTestCase

import app

class TestHelloApp(AsyncHTTPTestCase):
    def get_app(self):
        return app.make_app()

    def test_get(self):
        response = self.fetch('/')
        self.assertEqual(response.code, 200)
        self.assertEqual(response.body.decode(), '200 OK')

    def test_post(self):
        response = self.fetch('/', method="POST",
                              body=json.dumps({"key": "value"}))
        self.assertEqual(response.code, 200)
        self.assertEqual(response.body.decode(), '201 OK')

With that approach of installation asyncio loop application works fine (I mean I can do requests and I'm getting responses), but test like this failed with error:

======================================================================
FAIL: test_post (test_app.TestHelloApp)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/biceps/work/torn/.venv/lib/python3.6/site-packages/tornado/testing.py", line 380, in setUp
    self._app = self.get_app()
  File "/home/biceps/work/torn/test_app.py", line 8, in get_app
    return app.make_app()
  File "/home/biceps/work/torn/app.py", line 14, in make_app
    tornado.platform.asyncio.AsyncIOMainLoop().install()
  File "/home/biceps/work/torn/.venv/lib/python3.6/site-packages/tornado/ioloop.py", line 181, in install
    assert not IOLoop.initialized()
AssertionError

----------------------------------------------------------------------
Ran 2 tests in 0.006s

FAILED (failures=1)

Seems like loop that was installed by AsyncIOMainLoop().install() command is not stopped between tests, first test passed OK, but second always failed.

When I moved AsyncIOMainLoop().install() to start_app() method - tests are passed OK, but I'm worrying about that during test I use one event loop, but in real running app I use asyncio loop.

So, against that code tests are passed OK:

from tornado.platform.asyncio import AsyncIOMainLoop
import asyncio
import tornado.web


class MainHandler(tornado.web.RequestHandler):
    async def get(self, *args, **kwargs):
        self.write("200 OK")

    async def post(self, *args, **kwargs):
        self.write("201 OK")

def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),
    ], debug=True)

def start_app():
    AsyncIOMainLoop().install()
    app = make_app()
    app.listen(8888)
    loop = asyncio.get_event_loop()
    loop.set_debug(True)
    loop.run_forever()

Q: My question is - how to do write tests in that usecase correctly ? How to write tests with AsyncHTTPTestCase when Tornado app uses AsyncIOMainLoop ? Am I right with decision to make AsyncIOMainLoop().install() into start_app(), not in make_app() function ?

P.S. I've added self.io_loop.clear_instance() to tearDown() - it looks probably dirty but that works for case when AsyncIOMainLoop().install() called from make_app() code.

    def tearDown(self):
    self.io_loop.clear_instance()
    super().tearDown()

According to documentation I need to install AsyncIOMainLoop before startup application, not when I'm making app. documentation

from tornado.platform.asyncio import AsyncIOMainLoop
import asyncio
AsyncIOMainLoop().install()
asyncio.get_event_loop().run_forever()

So now I'm sure that proper way is using AsyncIOMainLoop installation into start_app() code.

So now my pattern code looks like:

web1.py

class MainHandler(tornado.web.RequestHandler):
    async def get(self, *args, **kwargs):
        await asyncio.sleep(1)
        return self.write("OK")

    async def post(self, *args, **kwargs):
        await asyncio.sleep(1)
        return self.write("OK")

def make_app():
    return tornado.web.Application([(r"/", MainHandler),],
                                   debug=False)

def start_app():
    from tornado.platform.asyncio import AsyncIOMainLoop
    import asyncio
    AsyncIOMainLoop().install()
    app = make_app()
    app.listen(8888)
    asyncio.get_event_loop().run_forever()


if __name__ == "__main__":
    start_app()

test_app.py

from tornado.testing import AsyncHTTPTestCase

import web1

class TestTornadoAppBase(AsyncHTTPTestCase):
    def get_app(self):
        return web1.make_app()

    def get_new_ioloop(self):
        """
        Needed to make sure that I can also run asyncio based callbacks in my tests
        """
        io_loop = tornado.platform.asyncio.AsyncIOLoop()
        asyncio.set_event_loop(io_loop.asyncio_loop)
        return io_loop

class TestGET(TestTornadoAppBase):
    def test_root_get_method(self):
        response = self.fetch("/")
        self.assertEqual(response.code, 200)
        self.assertEqual(response.body.decode(), 'OK')

    def test_root_post_method(self):
        response = self.fetch("/", method="POST", body="{}")
        self.assertEqual(response.code, 200)
        self.assertEqual(response.body.decode(), 'OK')

This pattern works as well, and during tests AsyncIOMainLoop is used. So I can use libraries those use asyncio loop. In my example there is asyncio.sleep() for example.