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

[Guido van Rossum]

[adding back the list]

On 3/13/07, Josiah Carlson <jcarlson at uci.edu> wrote:
> What if the thing to the right of 'as' were assigned different values
> depending on what it was?  If it were one name, it would be the
> exception (without traceback) "except ... as foo".  If it was a 3-tuple
> of names, it would get all three: "except ... as ecls, e, tb:". (with 2,
> maybe e and tb)  Anything else could raise a SyntaxError during the code
> generation phase.
>
> In cases where we get a tb, we can clear the traceback at the end of the
> except clause without needing to clear the exception, etc. Whether or
> not the traceback exists as an attribute of the exception would then
> become an implementation detail.  For cases where we pass an exception
> instance around, I would introduce a function called get_tb(e) (in
> either sys or traceback) that takes the exception object and returns the
> traceback. Whether or not it should print a warning unless the
> keyword-only argument of 'yes_I_know_I_need_to_clear_the_traceback' is
> provided and is True, or if it returns a weakref, I'll leave to someone
> else's discretion.

That sounds like too much complexity (both the syntax proposal and the
get_tb() API). With the new GC, clearing the traceback is really not
all that important any more except in the vicinity of buggy code that
expects that leaving a scope GC's its locals; apart from that it's
more of a performance issue than anything else, so I don't see why we
can't just have __traceback__ on the exception object.

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