def _num_difference(obj_a, obj_b):
    """Return the number of instance attributes present only in one object."""
    attrs_a = set(obj_a.__dict__)
    attrs_b = set(obj_b.__dict__)
    diff = attrs_a.symmetric_difference(attrs_b)
    privates = len([x for x in diff if x.startswith("_")])
    return len(diff) - privates