aiohttp dynamic resource in subapp prefix

example.py

#!/usr/bin/python3.7
# pytest -s test_subapps.py
# pip install pytest aiohttp pytest-aiohttp

from aiohttp import web
import pytest

def handle_1(request):
  return web.Response(text="OK 1")

def handle_2(request):
  key = request.match_info.get('key')
  return web.Response(text=f"OK {key}")

@pytest.fixture
def an_app():
  root = web.Application()
  root.router.add_get('/1', handle_1)
  child = web.Application()
  if False:
    # This works, it makes a "<DynamicResource  /child/{key}/2>"
    child.router.add_get('/{key}/2', handle_2)
    root.add_subapp('/child', child)
  else:
    # This breaks, it makes a "<PlainResource  /child/{key}/2>" 
    child.router.add_get('/2', handle_2)
    root.add_subapp('/child/{key}', child)
  print()
  print(root.router.routes()._routes)
  return root

@pytest.fixture
async def cli(test_client, an_app):
  client = await test_client(an_app)
  return client

async def test_this_does_not_work(cli):
  res1 = await cli.get('/1')
  res2 = await cli.get('/child/abc/2')

  print()
  print(await res1.text())
  print(await res2.text())
  assert res1.status == 200
  assert res2.status == 200