[Python-Dev] RE: PEP-317

[Michael Hudson]

Michael Chermside <mcherm@mcherm.com> writes:

> Greg Ewing:
>> Also while we're at it, how about storing the traceback in the
>> exception object, and getting rid of sys.exc_info() etc.?
>
> Guido:
>> That's a good idea, and I'd support a PEP on that topic more than PEP
>> 317 (which IMO is unnecessary).
>
> IMO the biggest advantage of PEP 317 is not what it does directly but the
> future enhancements it allows. If all exceptions really are objects, 
> (particularly if strengthened to say "instances of Exception or subclasses")
> then we can add new features to Exception. Moving the traceback into
> the object is one idea. Making Exception a new-style object is another
> (no need to rush, but it'll happen SOMEDAY, right?). Exception
> chaining (aka exception masking) is another.

But PEP 317 seems to regard removing string exceptions entirely
(something I support, possibly with a backward compatibility warning
laden shim) as a minor stepping stone on the way to removing implicit
instantiation (something I have a hard time caring about).

And as a random data point, I cannot see me stopping typing e.g.

raise TypeError, 'must set field "%s" to "%s", not "%s"'%(fname, reqt, gott)

until Python shouts at me each and every time I do it.  Some habits
die hard[1].

> The point is, without PEP 317, you can't do these things, because sometimes
> you use an object, and sometimes just mention a class (and sometimes just
> use a string). PEP 317 choose not to try to bite off anything bigger than
> fixing this so future enhancements would be possible. But we EVER want
> these kinds of enhancements, then start a PEP 317 style migration NOW,
> because degree of code breakage means that the migration time will be really
> slow. Deprecating (ideally with warnings) sooner is better.

I don't see how removing implicit instantiation makes this easier.

Cheers,
M.
... who would also like to see restartable exceptions, but isn't going
to mention that today, oh no ...

[1] I never got *into* the habit of using string exceptions, and in
    fact still don't quite understand what happens when you try and
    catch one (this is not an invitation for an explanation).

-- 
  ... but I guess there are some things that are so gross you just have
  to forget, or it'll destroy something within you.  perl is the first
  such thing I have known.              -- Erik Naggum, comp.lang.lisp