Python complaints

Gordon McMillan gmcm at hypernet.com
Thu Nov 25 14:16:12 CET 1999


Toby J Sargeant wrote:

> Just throwing this up in the air, and seeing what comes back...
> 
> class Acquire:
>    def __init__(self,lock):
>       self.lock=lock
>       lock.acquire()
> 
>    def __del__(self):
>       self.lock.release()
> 
> def function(mutex):
>   _a=Acquire(mutex)
>   # do stuff.
> 
> It relies on Acquire.__del__() being called as soon as _a goes
> out of scope, but that doesn't seem likely to change for the
> moment. (Of course doing this in JPython is a recipe for
> disaster... or at least extreme inefficiency)
> 
> It also has the advantage that you can exit the function in many
> ways (including by exception), and the lock will be released
> regardless.

Most unfortunately, exiting by exception will foul you up 
because the traceback will have a reference to the Acquire 
instance. You can clear it when you catch the exception, but 
since the consequences of forgetting to do that are both 
disastrous and mysterious, I think this idiom is best left to 
C++ (where I use it all the time).



- Gordon




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