[Python-Dev] PEP 340: Deterministic Finalisation (new PEP draft, either a competitor or update to PEP 340)

[Nick Coghlan]

Paul Moore wrote:
> On 5/8/05, Jp Calderone <exarkun at divmod.com> wrote:
> 
>>  If such a construct is to be introduced, the ideal spelling would seem to be:
>>
>>    for [VAR in] EXPR:
>>        BLOCK1
>>    finally:
>>        BLOCK2
> 
> 
> While I have not been following this discussion at all (I don't have
> the energy or time to follow the development of yet another proposal -
> I'll wait for the PEP) this does read more naturally to me than any of
> the other contortions I've seen passing by.

Given this for loop syntax:

   for VAR in EXPR:
       BLOCK1
   else:
       BLOCK2
   finally:
       BLOCK3

And these semantics when a finally block is present:

   itr = iter(EXPR1)
   exhausted = False
   try:
       while True:
           try:
               VAR1 = itr.next()
           except StopIteration:
               exhausted = True
               break
           BLOCK1
       if exhausted:
           BLOCK2
   finally:
       try:
           BLOCK3
       finally:
           itr_exit = getattr(itr, "__exit__", None)
           if itr_exit is not None:
               try:
                   itr.__exit__(TerminateBlock)
               except TerminateBlock:
                   pass

"Loop on this iterator and finalise when done" would be written:

   for item in itr:
       process(item)
   finally:
       pass

If you just want the finally clause, without finalising the iterator, you write 
it as you would now:

   try:
       for item in itr:
           process(item)
   finally:
       finalisation()

I like it - I'll update the PEP redraft to use it instead of the 'del' idea.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
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