aiohttp: how to add sub_app to sub_app?

Question

I need to add sub_app to sub_app. Here is my code

app = web.Application()

subapp = web.Application()
subapp.router.add_get('/page1/', handler)
app.add_subapp('/wt/', subapp)

subsubapp = web.Application()
subsubapp.router.add_get('/page2/', handler)
subapp.add_subapp('/sub/', subsubapp)

Why the page /wt/sub/page2/ is not found? I think that the path for subsubapp is a concatenation of /wt/ (sub_app) and /sub/ (subsubapp). Am I wrong? What is the rule?

Regards, Valery.

Answer 1

aiohttp version 2.3 has a subtle bug: it doesn't inform about improper sub application initialization order.

Sub-application logically is frozen after registering in parent, adding new routes to it is an error.

Not released yet aiohttp 3.0 raises explicit exception on app.add_subapp() call, see corresponding issue and pull request for more details.

To fix your snippet reorder route registering calls:

from aiohttp import web

async def handler(request):
    return web.Response(text="OK")

subsubapp = web.Application()
subsubapp.router.add_get('/page2/', handler)

subapp = web.Application()
subapp.router.add_get('/page1/', handler)
subapp.add_subapp('/sub/', subsubapp)

app = web.Application()
app.add_subapp('/wt/', subapp)

web.run_app(app, host='127.0.0.1', port=8080)

After this fetching http://127.0.0.1:8080/wt/sub/page2/ resources returns 200 OK.