Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail From: Ethan Furman Newsgroups: comp.lang.python Subject: Re: Static caching property Date: Mon, 21 Mar 2016 10:45:35 -0700 Lines: 39 Message-ID: References: <35a5c4206a0c40e584d62d5d37b068b3@activenetwerx.com> <56f0230e$0$1616$c3e8da3$5496439d@news.astraweb.com> , <89e865acae6941da978a0fb42c1df7b0@activenetwerx.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Trace: news.uni-berlin.de kt6puKYjrzWEYwQlh8wD+QcPP0ldB61iFNqkaLbCAmEA== Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.001 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'none:': 0.05; 'option,': 0.07; '*args):': 0.09; '@property': 0.09; 'descriptor': 0.09; 'from:addr:ethan': 0.09; 'from:addr:stoneleaf.us': 0.09; 'from:name:ethan furman': 0.09; 'message-id:@stoneleaf.us': 0.09; 'def': 0.13; '(but': 0.15; 'cleaner': 0.16; 'complicating': 0.16; 'object()': 0.16; 'received:io': 0.16; 'received:psf.io': 0.16; 'wrote:': 0.16; 'class,': 0.22; 'am,': 0.23; 'slightly': 0.23; 'import': 0.24; 'header:In-Reply-To:1': 0.24; 'header:User- Agent:1': 0.26; 'wonder': 0.27; '~ethan~': 0.29; "i'm": 0.30; 'skip:_ 10': 0.32; 'class': 0.33; 'option.': 0.33; 'correctly': 0.34; 'that,': 0.34; 'instance': 0.35; 'protocol': 0.35; 'something': 0.35; 'but': 0.36; 'to:addr:python-list': 0.36; 'subject:: ': 0.37; 'means': 0.39; 'test': 0.39; 'sure': 0.39; 'to:addr:python.org': 0.40; 'charset:windows-1252': 0.62; 'accessed': 0.66; 'cheap...': 0.84; 'self.value': 0.84 User-Agent: Mozilla/5.0 (X11; Linux i686; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 In-Reply-To: <89e865acae6941da978a0fb42c1df7b0@activenetwerx.com> X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Xref: csiph.com comp.lang.python:105381 On 03/21/2016 10:03 AM, Joseph L. Casale wrote: >> One solution is to use descriptor protocol on the class, which means >> using a metaclass. I'm not sure it's the best option, but it is an >> option. > > I will look at that, I wonder if however I am not over complicating it: > > class Foo: > _bar = None > @property > def expensive(self): > if Foo._bar is None: > import something > Foo._bar = something.expensive() > return Foo._bar > > Somewhat naive, but a test with if is pretty cheap... A slightly cleaner approach (but only slightly): class Cache(object): _sentinal = object() def __init__(self, expensive_func): self.value = self._sentinal self.func = expensive_func def __get__(self, *args): if self.value is self._sentinal: self.value = self.func() return self.func() The advantages: - only one location in the class - works correctly whether accessed via class or instance - clue as to functionality in the name -- ~Ethan~