Re: [SQLObject] caching an expensive property

[Oleg B. <ph...@ph...>]

On Wed, Dec 10, 2014 at 01:09:50AM +0100, "Fetchinson ." <fet...@go...> wrote:
> On 12/10/14, Oleg Broytman <ph...@ph...> wrote:
> > you'd better implement your own
> > caching with proper locking.
> 
> Well, that was exactly the question, how would I do that? :)
> Precisely for the reason you mention above I was wary of using a more
> or less standard caching decorator. Can't I attach the computed value
> to the instance itself somehow by setting a new property?

   The classical approach is:

    def __init__(self):
        self.__cache = None
        self.__cache_lock = Lock()

    def _get_value(self):
        if self.__cache is not None:
            return self.__cache
        self.__cache_lock.acquire()
        try:
            if self.__cache is not None: # Calculated in another thread
                return self.__cache
            self.__cache = ...do expensive calculation once...
            return self.__cache
        finally: # finally works both for exceptions and returns
            self.__cache_lock.release()

Oleg.
-- 
     Oleg Broytman            http://phdru.name/            ph...@ph...
           Programmers don't die, they just GOSUB without RETURN.