[Python-Dev] PEP 343 - next steps

[Nick Coghlan]

Phillip J. Eby wrote:
> Ow.  My head hurts.  :)  Seriously, though, wouldn't it make more sense 
> to put the 'with async_exceptions_blocked()' in the __init__ or 
> __enter__ of 'critical_resource'?  Or am I still missing something?

Heck, I *suggested* the trick, and still had to look at it a half 
dozen times, including frontways, and backways and upside down in 
order to satisfy myself that it actually addressed all the issues 
Guido and I had been talking about.

Anyway, the kicker is that (in the rare cases where it matters), you 
want asynchronous exceptions blocked before EXPR gets evaluated, and 
then unblocked again after the try/finally has been entered, but 
before BLOCK starts evaluating.

I believe the only real way to do that deterministically is to do the 
blocking and unblocking of asynchronous exceptions inline - when the 
asynchronous exception occurs, either the critical resource hasn't 
been acquired yet, or we are inside a try/finally that will ensure its 
release.

Trying to push the blocking/unblocking inside a template like 
safe_acquisition actually results in the loss of that determinism, 
since it may be the case that safe_acquisition itself is not finalised 
properly (the asynchronous exception goes off prior to entry of the 
relevant try/finally block). However, the generator object's __del__ 
method will clean things up eventually.

Anyway, the main point is that with statements won't make any more 
guarantees about this than try/finally statements do - but they do 
provide the tool to deal with it reasonably cleanly when it matters 
(most of the time the asynchronous error will indicate program 
shutdown, and the remote possibility of misbehaviour won't really 
matter in practice).

Cheers,
Nick.

P.S. I've included this trick as part of the Wiki discussion, meaning 
my last PEP 343 question has been well and truly dealt with.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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             http://boredomandlaziness.blogspot.com