For Python 3, I've added optional type checking based on function annotations and decorators to the Ensure library: https://github.com/kislyuk/ensure#enforcing-function-annotat....

    @ensure_annotations
    def f(x: int, y: float) -> float:
        return x+y

    f(1, 2.3)
    >>> 3.3
    f(1, 2)
    >>> ensure.EnsureError: Argument y to <function f at 0x109b7c710> does not match annotation type <class 'float'>

I think it works better than other approaches mentioned here because it (1) doesn't repeat the contents of the function signature, (2) doesn't install any magic system-wide hooks (and the attendant performance issues), and (3) is completely optional.