new_contents: import base64
  from cryptography import fernet
  from aiohttp import web
  from aiohttp_session import setup, get_session
  from aiohttp_session.cookie_storage import EncryptedCookieStorage
  
  
  def flash(request, message):
      request.setdefault('flash_outgoing', []).append(message)
  
  
  def get_messages(request):
      return request.pop('flash_incoming')
  
  
  async def flash_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
  
  
  async def flash_handler(request):
      flash(request, 'You have just visited flash page')
      return web.HTTPFound('/')
  
  
  async def handler(request):
      text = 'No flash messages yet'
      messages = get_messages(request)
      if messages:
          text = 'Messages: {}'.format(','.join(messages))
      return web.Response(text=text)
  
  
  def make_app():
      app = web.Application()
      # secret_key must be 32 url-safe base64-encoded bytes
      fernet_key = fernet.Fernet.generate_key()
      secret_key = base64.urlsafe_b64decode(fernet_key)
      setup(app, EncryptedCookieStorage(secret_key))
      app.router.add_get('/', handler)
      app.router.add_get('/flash', flash_handler)
      # Install flash middleware
      app.middlewares.append(flash_middleware)
      return app
  
  
  web.run_app(make_app())
  
  
subject: Add example of how to use session to flash messages between requests (#270)