On Tue, 01 Aug 2017 11:05:38 -0400, Terry Reedy wrote: > On 8/1/2017 7:06 AM, Matt Wheeler wrote: >> On Tue, 1 Aug 2017 at 02:32 Terry Reedy <tjre...@udel.edu> wrote: >> >>> On 7/31/2017 7:31 PM, t...@tomforb.es wrote: >>>> As part of the Python 3 cleanup in Django there are a fair few uses >>>> of >>> @functools.lru_cache on functions that take no arguments. >>> >>> This makes no sense to me. If the function is being called for >>> side-effects, then it should not be cached. If the function is being >>> called for a result, different for each call, calculated from a >>> changing environment, then it should not be cached. (Input from disk >>> is an example.) If the function returns a random number, or a >>> non-constant value from an oracle (such as a person), it should not be >>> cached. If the function returns a constant (possible calculated >>> once), then the constant should just be bound to a name (which is a >>> form of caching) rather than using the overkill of an lru cache. What >>> possibility am I missing? >>> >>> >> A function which is moderately expensive to run, that will always >> return the same result if run again in the same process, and which will >> not be needed in every session. > > In initialization section: > answer = None > > Somewhere else: > answer = expensive_calculaton() if answer is None else answer > > The conditional has to be cheaper than accessing lru cache. There can > be more than one of these.
One conditional has to be cheaper than accessing lru cache. Debugging the errors from forgetting to wrap every single reference to "answer" in a "if answer is None" test is not so cheap. Maintaining twice as much state (one function plus one global variable, instead of just one function) is not so cheap. Sometimes paying a small cost to avoid having to occasionally pay a large cost is worth it. This is effectively the "Only Once" decorator pattern, which guarantees a function only executes once no matter how often you call it. -- “You are deluded if you think software engineers who can't write operating systems or applications without security holes, can write virtualization layers without security holes.” —Theo de Raadt -- https://mail.python.org/mailman/listinfo/python-list
