[Python-3000] exception info [was: Discussions with no PEPs]

[Guido van Rossum]

On 3/12/07, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Guido van Rossum wrote:
> > I'm still hoping we can get agreement that storing the traceback on
> > the exception object is the way to go. Only a handful of modules using
> > pre-baked exception objects would have to be fixed, and likely those
> > were doing it out of a misunderstanding of the performance
> > characteristics.
>
> One more thought on this -- do we still think it's a
> good idea to make it possible to raise and catch an
> exception in C code without instantiating anything?

I don't see it's contradictory to the proposal.

> If so, how does this interact with the idea of
> attaching the traceback to the exception?

The internal mechanism for bubbling an exception up the stack until an
except clause catches it could continue to use the (class, instance,
traceback) triple, and if raise is passed a class instead of an
instance, the instance would be NULL; when forced to instantiate the
exception, the traceback collected up to that point is attached to it.
If an instance was passed to raise, the __traceback__ pointer in the
instance is updated each time we leave a stack frame. I think this
means the __traceback__ attribute needs to correspond to a C-level
slot in the BaseException instance lay-out.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)