[Python-3000] self-contained exceptions

[Phillip J. Eby]

At 09:11 PM 1/3/2007 -0800, Guido van Rossum wrote:
>On 1/3/07, Phillip J. Eby <pje at telecommunity.com> wrote:
>>Or perhaps translate blocks of the form:
>>
>>      except ExcType, e:
>>          # body
>>
>>to:
>>
>>      except ExcType, e:
>>          try:
>>              # body
>>          finally:
>>              del e
>>
>>This won't stop you from creating a cycle explicitly, of course, but it
>>would ensure that the simple cases would be cycle-free.
>
>+1!
>
>I used to dislike this because there are use cases for letting the
>exception survive the except clause, but I think I can get used to it,
>and it seems the most straightforward solution of all that I've seen.
>We could completely get rid of sys.exc_info()! I think we have a
>winner here.

Actually, on second thought it occurs to me that the above code isn't a 
100% correct translation, because if "body" contains its own "del e", then 
it will fail.  So a pure translation strategy isn't really practical, 
unfortunately.  We'd have to generate something like:

     if 'e' in locals():
         del e

or add a new bytecode that's equivalent to that.


>Explicitly created cycles are no big deal IMO -- these are no worse
>than current code that explicitly stores sys.exc_info()[2].

True, but now they get that overhead simply by storing the exception 
object.  On the other hand, I suppose code that might store a lot of 
exception objects will just have to be careful to clear tracebacks it 
doesn't need.