I say confine carry out to delis, outdoor farmer markets, traveling festivals, supermercato, and smaller grocers and other stores that specialize in putting food in bags and handing it to you. That's a sure thing!
It's called PORTA VIA (vee ah), and a very common practice at the above listed merchants. But Not necessarily at sit-down restaurants.
Now I have seen locals in small towns go into a restaurant, walk straight to the kitchen, and carry out food in large covered tins, but when I said the magic words after a sit-down meal at the same place, the waittress thought I told her to throw away the leftovers, and that's exactly what she did!
I've Not seen the big tin carry out in cities, but, who knows, it may occur, maybe by a side door or back door! It doesn't hurt to ask, only I would say "POSSO PORTARE VIA?" So I'm asking may I? Is it possible? Can I do this?