[Python-Dev] Proposed addition to threading module - released

Mon Apr 24 21:10:29 CEST 2006

[Guido]
> Actually, what Nick describes is *exactly* how one should write code
> using a condition variable:
>> LOCK
> while nothing to do:
> UNLOCK
> wait for the condition variable (or sleep, or whatever)
> LOCK
> # here we have something to do with the lock held
> remove the to-do item
> UNLOCK
>> except that the outer LOCK/UNLOCK pair should be using a try/except
> and the inner UNLOCK/LOCK pair should too. I don't see how you can do
> this easily by rewriting the code; the rewrite would be considerably
> ugly (or requires a GOTO :-).

That didn't make much sense to me. If you're using a condition
variable `cv`, the way that should be written is:
 cv.acquire()
 try:
 while nothing to do:
 cv.wait() # which unlocks on entry, and locks before return
 do something
 finally:
 cv.release()
IOW, there is no "inner UNLOCK/LOCK" for the user to worry about
(although the implementation of wait() has to worry about it).
Does
 with cv:
work to replace the outer (== only) acquire/try/finally/release dance?
 If so, that seems like plenty to me.