Optional arguments are almost always trailing arguments:

    dumps(obj)
    dump(obj, fp)

Might be implemented (in a Pythonic way) by the same function:

    fn(obj, fp=None):
        ...

Another way to think about it is that you're dumping from obj to fp. The thing I always get backwards is C's memcpy(dst, src, len) which I've always thought has its first two arguments in the wrong order.