code: async def get_auth(request):
      
  
      auth_val = request.get(AUTH_KEY)
      if auth_val:
          return auth_val
  
      auth_policy = request.get(POLICY_KEY)
      if auth_policy is None:
          raise RuntimeError('auth_middleware not installed')
  
      request[AUTH_KEY] = await auth_policy.get(request)
      return request[AUTH_KEY]
docs: Returns the user_id associated with a particular request.
  
      Args:
          request: aiohttp Request object.
  
      Returns:
          The user_id associated with the request, or None if no user is
          associated with the request.
  
      Raises:
          RuntimeError: Middleware is not installed