[Python-Dev] Compilation of "except FooExc as var" adds useless store

[Chris Angelico]

On Mon, Jan 7, 2019 at 2:27 AM Serhiy Storchaka <storchaka at gmail.com> wrote:
> > The "store None" behavior can be traced down to introduction of the
> > "del except target var" behavior back in 2007:
> > https://github.com/python/cpython/commit/b940e113bf90ff71b0ef57414ea2beea9d2a4bc0#diff-cb296cc5109f5640ff3f6d7198a6abeeR1999
> >
> > There's no clear explanation why it's done like that, so probably an
> > artifact of the initial implementation. Note that even
> > https://github.com/python/cpython/commit/520b7ae27e39d1c77ea74ccd1b184d7cb43f9dcb
> > which did quite a bunch of refactoring to "except" implementation, and
> > reformatted this code, otherwise left it in place.
>
> Because there is a reason for such code.

What reason though??

> See issue1631942 [1] and the thread with the subject "self-contained
> exceptions" on the Python-3000 mailing list [2] for the rationale.
>
> In short, the code
>
>      try:
>          1/0
>      except Exception as e:
>          del e
>
> should work.

But obviously not, because the code was specifically written to set it
to None first. It's not the sort of thing that can be done
accidentally, is it?

ChrisA