calculator

[sparkz_alot]

as in your main() function, use a while statement

def sub():
    while True:
        try:
            num1 = int(input("Give a number: "))
            num2 = int(input("Give a number: "))
        except (TypeError, ValueError):
            print("This input is invalid.")
            # sub()
            continue
        else:
            return num1, num2

You should also account for division by zero (selections 4, 5 and 6).  You do not wish to use decimal numbers?