aiohttp flash messages

flash.py

from aiohttp_session import get_session


def flash(request, message):
    request.setdefault('flash_outgoing', []).append(message)


async def context_processor(request):
    return {
        'get_flashed_messages': lambda: request.pop('flash_incoming')
    }


async def middleware(app, handler):
    async def process(request):
        session = await get_session(request)
        request['flash_incoming'] = session.pop('flash', [])
        response = await handler(request)
        session['flash'] = (request.get('flash_incoming', []) +
                            request.get('flash_outgoing', []))
        return response
    return process

Note that this unnecessarily updates the session on each HTTP request even if the flash hasn't been used. This leads to a lot of redundant database update or Set-Cookie headers.

Also, the flashed messages are lost if the handler raises web.HTTPFound.