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.
