Not only that, but there's simply no guarantees. You can abuse a function in any way you see fit in Python, and the only one that suffers is your runtime sever

Not only that, but there's simply no guarantees. You can abuse a function in any way you see fit in Python, and the only one that suffers is your runtime sever :( Optional types in 3.5 look awesome - but i don't want to lose duck typing. I want Go-interfaces in Python.