Python 3 actually does let you annotate function arguments, although it's with an arbitrary expression rather than explicitly being a type. So this is valid:

Python 3 actually does let you annotate function arguments, although it's with an arbitrary expression rather than explicitly being a type. So this is valid: def haul(item: Haulable, *vargs: PackAnimal) -> Distance: As is this: def thing(something: "A string!") -> "does stuff": It's also not explicitly checked, although if you felt like it you could probably write a metaclass to enforce type checking. (Though I imagine to do so would be rather un-pythonic)