[Python-3000] Problem with grammar for 'except'?

[Guido van Rossum]

Someone please fix the PEP. There are very good reasons for *not*
allowing "except X, Y:" to have a meaning -- if 2.x code somehow
accidentally ended up in the 3.0 world without having been run through
2to3, it would silently perturb the meaning in the most confusing way.
That's why the implementation got it right.

--Guido

On Sun, Oct 5, 2008 at 7:14 PM, wesley chun <wescpy at gmail.com> wrote:
> On Thu, Sep 4, 2008 at 12:36 PM, Guido van Rossum <guido at python.org> wrote:
>> On Wed, Sep 3, 2008 at 9:25 PM, Raymond Hettinger <python at rcn.com> wrote:
>>> [Brett]
>>>> I gave a talk last night at the Vancouver Python users group on
>>>> 2.6/3.0, and I tried the following code and it failed during a live demo:
>>>>
>>>>  >>> try: pass
>>>>  ... except Exception, Exception: pass
>>>>   File "<stdin>", line 2
>>>>     except Exception, Exception: pass
>>>>                                ^
>>>>  SyntaxError: invalid syntax
>>>>
>>>> Now from what I can tell from PEP 3110, that should be legal in 3.0.
>>>> Am I reading the PEP correctly?
>>>
>>> Don't think so.
>>> The parens are necessary for a tuple of exceptions
>>> lest it be confused with the old "except E, v" syntax
>>> which meant "except E as e".
>>>
>>> Maybe in 3.1, the paren requirement can be dropped.
>>
>> I would wait longer -- until well after the 2.x line is dead and
>> buried. It will take some time for every Python user to train their
>> Python fingers not to type "except E, v:" and we don't want people who
>> are late in migrating inserting bugs like this in their first 3.x program.
>
>
> it's probably a good idea to leave the paren requirement in there, but
> i just reread the PEP myself, and it appears as though no parens is
> actually supported, specifically: "except AttributeError, os.error:"
> here:
>
> http://www.python.org/dev/peps/pep-3110/#grammar-changes
>
> also, and granted this is older info, Guido's 2006 talks seem to hint
> this as well:
>
> - change except clause syntax to except E1, E2, E3 as err:
>    - this avoids the bug in except E1, E2: # meant except (E1, E2)
>
> from both of these:
>
> ACCU - Apr 2006 (slide 11)
> http://www.python.org/doc/essays/ppt/accu2006/Py3kACCU.ppt
>
> Vancouver Python Workshop - Aug 2006 (slide 13)
> http://www.vanpyz.org/conference/2006/proceedings/MarygX/Py3KVanPyz.ppt
>
> while we can't change the past, we can/should at least update the PEP
> as well as the current 2.6 and 3.0 docs to specifically state that the
> parens are required (for now) *and* give an example usage.
>
> cheers,
> -- wesley
>
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)