[Python-3000] How far to go with cleaning up exceptions

[Brett Cannon]

On 3/2/07, Collin Winter <collinw at gmail.com> wrote:
> On 3/1/07, Brett Cannon <brett at python.org> wrote:
> > I spent my sprint time cleaning up exceptions for Py3K and Guido
> > suggested I run some things by the group to make sure they don't make
> > transitioning from 2.6 too difficult.  After adding the proper
> > restrictions in terms of what can and cannot be raised or caught, I
> > began cleaning up BaseException's API per PEP 352.
> >
> > First thing I did was remove slicing.  That wasn't too bad since I
> > just got exceptions about the lack of __getitem__ and went to the
> > affected line and tossed in '.args' between the index and the
> > exception.  Not sure if 2to3 can handle this as it would need to
> > realize when an exception is within scope and then insert '.args' when
> > it is be indexed on.
>
> 2to3 currently cannot handle this. 2.6's "py3k-compat" mode could
> handle it easily, though.
>

Yep.  I exchanged some emails with Guido privately and this was the
conclusion reached.

> > The next thing I did was strip out the use of '.args' and make
> > BaseException take a single argument that gets assigned to 'message'.
> > The fruits of this labour are in the p3yk_no_args_on_exc branch.  This
> > one turned out to be a pain in the rear.
> [snip]
>
> I took at stab at this same conversion a few weeks ago and had the
> exact same experience. I ended up resolving my frustration by applying
> "svn revert" and working on something else : )
>

=)  The people at the sprint (especially days 2 and 3) got to hear my
frustrations constantly.

> I think this will shape up to be one of the harder parts of porting
> from 2.x to 3.0.
>

I agree, which is why I am asking if people are still supportive of
this?  Are you, Collin?

> > As for transforming the 'args' accesses to something else,
> > I guess 2to3 could automatically do 'args[0]' accesses to 'message',
> > but anything else will need to be changed by hand.
>
> There's very little 2to3 can do on this front. I think 2.6's
> py3k-compat flag is going to be the best bet for this particular
> conversion.

I think the slice removal is easy.  But getting rid of 'args' and
making BaseException's constructor only take a single argument is not
painless.  But a warning should help get this done in a reasonable
fashion.

-Brett