I believe now it is my turn to say you're spewing nonsense. Have you ever used a language with a type system? Because this whole idea of "testing external data to see if it is the right type" doesn't really make sense.

At the lowest layer, generally, external data is an array of bytes. It has no other type you could "test". However, you can decode or parse it into a form that makes sense for your program. And why couldn't I cleanly recover from parse errors? (And how do you think Python does it?)