[Python-Dev] PEP 343 - Abstract Block Redux

[Nick Coghlan]

Guido van Rossum wrote:
> [Nick Coghlan]
> 
>>Also, the call to __enter__() needs to be before the try/finally block (as it is
>>in PEP 310). Otherwise we get the "releasing a lock you failed to acquire" problem.
> 
> 
> I did that on purpose. There's a separate object ('abc' in the
> pseudo-code of the translation) whose __enter__ and __exit__ methods
> are called, and in __enter__ it can keep track of the reversible
> actions it has taken.
> 
> Consider an application where you have to acquire *two* locks regularly:
> 
>     def lockBoth():
>         got1 = got2 = False
>         lock1.acquire(); got1 = True
>         lock2.acquire(); got2 = True
>         yield None
>         if got2: lock2.release()
>         if got1: lock1.release()
> 
> If this gets interrupted after locking lock1 but before locking lock2,
> it still has some cleanup to do.
> 
> I know that this complicates simpler use cases, and I'm not 100% sure
> this is the right solution; but I don't know how else to handle this
> use case.
> 

If we retained the ability to inject exceptions into generators, this would be 
written with the extremely natural:

   @with template:
   def lockboth():
     lock1.acquire()
     try:
         lock2.acquire()
         try:
             yield
         finally:
             lock2.release()
     finally:
         lock1.release()

Or, even more simply:

    @with_template:
    def lockboth():
        with lock1:
            with lock2:
                yield

I think Fredrik's intuition is on to something - PEP 343 has scaled the idea 
back *too* far by throwing away the injection of exceptions into generator 
templates, when the only major objection was to the looping nature of the proposal.

Cheers,
Nick.

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