GvR> There was some discussion that concluded that it was a useful GvR> feature (e.g. because the Exception class defines a standard GvR> signature and some standard attributes)
But most of the (early) built-in exception interfaces were chosen for backwards compatibility with the old string-based standard exceptions, so I'm not sure how useful they are as a specification. E.g. Exception.__init__() essentially takes a *args and assigns the tuple to exc.args, and that's about it. Maybe in a perfect world that's still a useful signature, I don't know.
Well, it could certainly grow more standard stuff (like a traceback pointer) and that would be more useful if there was a common base class to inherit it from. --Guido van Rossum (home page: http://www.python.org/~guido/)