[Python-3000] self-contained exceptions

Tue Jan 9 22:10:15 CET 2007

Guido van Rossum wrote:
> On 1/8/07, Collin Winter <collinw at gmail.com> wrote:
>> FWIW, I wasn't aware that things like "except T, (a, b, c):" were
>> possible until I started work on this patch; I thought <name> was all
>> you _could_ use.
>> And, given the secial semantics for this position, I would support
> changing the syntax to *only* allow a single simple name there -- not
> even x.y.z. Thsh should handle 99.9% of existing code anyway.
>> (And yes, if someone wants to add the 'as' keyword at the same time
> that would be great!

With such changed semantics (only allow simple name, simple name gets 
deleted at the end of the except: block) I would be strongly in favour of 
only having the changed semantics when the 'as' keyword is used.
This would definitely make the warning about usage of old-style exceptions 
much easier - no need to try to determine if the exception name is used 
outside of the block - just check for an exception clause that doesn't use 
'as'.
Haven't checked the patch, but does it successfully delete the name in the 
case that another exception is thrown inside the except block?
 try:
 raise Exception()
 except Exception as e:
 try:
 raise Exception()
 except Exception as e2:
 pass
Also, what are the semantics of:
 try:
 raise Exception()
 except Exception as e:
 try:
 raise Exception()
 except Exception as e:
 pass
 print e
and
 try:
 raise Exception()
 except Exception as e:
 try:
 pass
 except Exception as e:
 pass
 print e
As I understand the proposed semantics, in the first case a NameError will 
be thrown at the 'print e' line in the first case, but not in the second 
case, because we never enter the second except: block. For consistency, 
maybe instead the semantics be that any name declared for catching an 
exception be deleted at the end of the try/except/else/finally clause i.e.
 try:
 pass
 except AttributeError as e1:
 pass
 except NameError as e2:
 pass
is equivalent to:
 try:
 try:
 pass
 except AttributeError, e1:
 pass
 except NameError, e2:
 pass
 finally:
 e1 = None
 e2 = None
 del e1
 del e2
Tim Delaney 