[Python-3000] removing exception .args

[Brett Cannon]

On 7/21/07, Andrew Dalke <dalke at dalkescientific.com> wrote:
> Posting here a expansion of a short discussion I had
> after Guido's keynote at EuroPython.  In this email
> I propose eliminating the ".args" attribute from the
> Exception type.  It's not useful, and supporting it
> correctly is complicated enough that it's often not
> supported correctly
>

This was originally proposed in PEP 352.  This was the reason for the
existence of the 'message' attribute as introduced in Python 2.5..  At
PyCon 2007 I actually removed 'args' (see the p3yk_no_args_on_exc
branch in svn: http://svn.python.org/view/python/branches/p3yk_no_args_on_exc/).

But after making everyone at PyCon suffer through my swearing and
frustration and talking with python-dev (and thus should be in the
python-dev/python-3000 archives), the decision was made to not remove
it (which is why 'message' is deprecated in Python 2.6).  This was
because the removal at the C level is very painful.  There are many
places within the code where a tuple is passed to various C functions
that expect that tuple to be treated as multiple arguments to the
exception constructor.

But changing the semantics of a C function has already been labeled a
no-no.  So one would have to remove the C functions that construct
exceptions with arguments and use a new one that only expects a single
argument so not to have unexpected semantics.  That sucks because
those functions are all over.

In the branch I just stuck the tuple into the 'message' attribute, but
that caused its own issues as output was now a little funky since
everything was considered a tuple, including single arguments.

So while I totally understand the desire to ditch 'args' and just have
'message', doing so thoroughly and in any reasonable way that is not
painful is not easy thanks to the C API.

-Brett