In the US, it is extremely common to ask for a doggy bag to take any unfinished food home with you. After all, sometimes you just can't finish your whole meal, and there is no reason to leave it behind at the restaurant if you paid for it. You can take it home and have a snack for later. However, this is actually pretty uncommon in many countries outside of the US.
Although the restaurant probably won't tell you no, they will be a bit surprised and maybe even assume that you are American as a result. But who cares? It's worth having the snacks!