What you want is to acquire a lock if the cache is empty, check if another thread has filled the cache while you where waiting on the lock, call the function, fill the cache and return.
I don’t see how you could implement that with two independent decorators without locking all accesses (before `once`) or having the chance that it’s called more than once (after `once`). Tom > On 29 Apr 2020, at 08:15, Andrew Barnert via Python-ideas > <python-ideas@python.org> wrote: > > On Apr 28, 2020, at 16:25, Steven D'Aprano <st...@pearwood.info> wrote: >> >> On Tue, Apr 28, 2020 at 11:45:49AM -0700, Raymond Hettinger wrote: >> >>> It seems like you would get just about everything you want with one line: >>> >>> once = lru_cache(maxsize=None) >> >> But is it thread-safe? > > You can add thread safety the same way as any other function: > > @synchronized > @once > def spam(): > return 42 in a slow and non-thread-safe and non-idempotent way and > also launch the missiles the second time we’re called > > Or wrap a with lock: around the code that calls it, or whatever. > > Not all uses of once require thread safety. For the really obvious example, > imagine you’re sharing a singleton between coroutines instead of threads. And > if people are really concerned with the overhead of lru_cache(maxsize=None), > the overhead of locking every time you access the value is probably even less > acceptable when unnecessary. > > So, I think it makes sense to leave it up to the user (but to explain the > issue in the docs). Or maybe we could add a threading.once (and > asyncio.once?) as well as functools.once? > > > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/RSJLUF4R6TM3HSILZYWGB366WUHQT755/ > Code of Conduct: http://python.org/psf/codeofconduct/ _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/LZPL7SIIEDUZW7P6CTRYJCXZVQOJJL24/ Code of Conduct: http://python.org/psf/codeofconduct/
