Duck typing. A Python object doesn't have to have a specific type, just the attribute(s) you want. You'll find a lot of "hasattr(obj, x)" throughout any large Python code base. It's nice to not have to worry about creating single-method/single-attribute interfaces and dot them all over the place.
You could specify that your method takes an object which has method XYZ accepting an int as its parameter and returning another int -- a kind of "anonymous" interface, I guess. Personally I don't like this kind of stuff because there's no guarantee that two objects with a convert method do the same thing when that method is called.
