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Groups > comp.lang.python > #75901

Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager

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On 8/8/2014 12:16 PM, Chris Angelico wrote:
> On Sat, Aug 9, 2014 at 2:05 AM, Neil D. Cerutti <neilc@norwich.edu> wrote:
>> Perhaps defer release, a la a common Go pattern:
>>
>> with contextlib.ExitStack() as stack:
>>      acquired = lock.acquire(blocking=False)
>>      if acquired:
>>          stack.callback(lock.release)
>>          do_stuff
>
> There's a race condition in that - an unexpected exception could
> happen between those two. Are you able to set the callback to be a
> "release if acquired" atomic operation?

Doesn't any natural looking use of blocking=False suffer from the same 
race condition? What's the correct way to use it?

Here's another attempt at context managing:

@contextlib.contextmanager
def release_if_acquired(lock, blocking=True, timeout=-1):
     acquired = lock.acquire(blocking, timeout)
     if acquired:
         yield acquired
         lock.release()
     else:
         yield acquired

with release_if_acquired(lock, blocking=False) as acquired:
     if acquired:
         do_stuff

-- 
Neil Cerutti

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Specifying `blocking` and `timeout` when acquiring lock as a context manager cool-RR <ram.rachum@gmail.com> - 2014-08-08 04:51 -0700
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager Ethan Furman <ethan@stoneleaf.us> - 2014-08-08 06:25 -0700
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager "Neil D. Cerutti" <neilc@norwich.edu> - 2014-08-08 12:05 -0400
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager Chris Angelico <rosuav@gmail.com> - 2014-08-09 02:16 +1000
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager "Neil D. Cerutti" <neilc@norwich.edu> - 2014-08-08 14:35 -0400
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager "Neil D. Cerutti" <neilc@norwich.edu> - 2014-08-08 14:57 -0400
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager Chris Kaynor <ckaynor@zindagigames.com> - 2014-08-08 12:07 -0700
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager Ian Kelly <ian.g.kelly@gmail.com> - 2014-08-08 13:21 -0600
  Re: Specifying `blocking` and `timeout` when acquiring lock as a context manager Chris Angelico <rosuav@gmail.com> - 2014-08-09 08:57 +1000

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