Ah, you are right, I wasn't looking at the full context of the diff when I made the comment about backward compatibility.  So ignore that part :)

On the other hand, exposing perferred_auth on the class would be a simple API for allowing auth extension (since one really needs to subclass to add an authobject, given that it needs access to self.user and self.password).  But, like I said before, that can be a separate issue.