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Looks like both of answers are outdated.
Now (python 3.12 + aiohttp 3.10.9) it may be simply done with middleware:
from aiohttp import web, web_exceptions
from typing import Callable

# here is our middleware that will handle all errors including 404
# and rewrite responses
@web.middleware
async def custom_error_handling_middleware(request: web.Request, handler: Callable):
    try:
        resp: web.Response = await handler(request)
    # here we are catching aiohttp related exception
    except web_exceptions.HTTPException as e:
        # since HTTPException childs are response-ready it may be return as response later
        resp = e
        # if status code of our exception is 404
        # we will sutomize it as we want (update body and headers in my case)
        if resp.status_code == web_exceptions.HTTPNotFound.status_code:
            resp.text = '<html><body><code>How did you find me? ≧☉_☉≦</code></body></html>'
            resp.content_type = 'text/html'
            resp.charset = 'utf-8'
    # here we are generating custom page for a specific non aiohttp related exception
    # (didn't asked in the question but added as bonus)
    except ZeroDivisionError:
        resp = web_exceptions.HTTPInternalServerError(
            text='<html><body><code>Mamma mia! (╯`Д´)╯︵ ┻━┻</code></body></html>',
            content_type = 'text/html',
        )
    # here we are generating custom page for any unexpected exception
    # (didn't asked in the question but added as bonus)
    except Exception as e:
        resp = web_exceptions.HTTPInternalServerError(
            text='<html><body><code>Completely unexpected error ¯\\(°_o)/¯</code></body></html>',
            content_type='text/html',
        )
    return resp

# correct view
async def hello(_: web.Request):
    return web.Response(
        text = '<html><body><code>Hello world ( っ˶´ ˘ `)っ</code></body></html>',
        content_type='text/html',
        charset='utf-8',
    )

# view that will respond an expected error
async def divizion_error(_: web.Request):
    1/0
    return web.Response(text='response will never happen')

# view that will respond an unexpected error
async def unexpected_error(_: web.Request):
    return web.Response(text=unbound_variable_for_example)

# standard functions to make and run a demo app
async def create_app():
    app = web.Application(middlewares=[custom_error_handling_middleware])
    app.add_routes([
        web.get("/div", divizion_error, name='divizion_error_view'),
        web.get("/unex", unexpected_error, name='unexpected_500_view'),
        web.get("/", hello, name='only_existant_view'),
    ])
    return app

if __name__ == '__main__':
    app = create_app()
    web.run_app(app, port=8888)
After adding the custom_error_handling_middleware, How did you find me will be shown as standard 404 page
79073848
