Re: Static caching property

From	Ethan Furman <ethan@stoneleaf.us>
Newsgroups	comp.lang.python
Subject	Re: Static caching property
Date	2016-03-21 10:45 -0700
Message-ID	<mailman.452.1458582319.12893.python-list@python.org> (permalink)
References	<35a5c4206a0c40e584d62d5d37b068b3@activenetwerx.com> <mailman.446.1458576961.12893.python-list@python.org> <56f0230e$0$1616$c3e8da3$5496439d@news.astraweb.com> <CAPTjJmqvu4qMpcbR-g5QZG6neuJVcHrKLYLDFe5QC_QBQbd2-g@mail.gmail.com> <89e865acae6941da978a0fb42c1df7b0@activenetwerx.com>

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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~

Thread

Re: Static caching property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-21 10:15 -0600
  Re: Static caching property Steven D'Aprano <steve@pearwood.info> - 2016-03-22 03:36 +1100
    Re: Static caching property "Joseph L. Casale" <jcasale@activenetwerx.com> - 2016-03-21 16:49 +0000
    Re: Static caching property Chris Angelico <rosuav@gmail.com> - 2016-03-22 03:54 +1100
    Re: Static caching property "Joseph L. Casale" <jcasale@activenetwerx.com> - 2016-03-21 17:03 +0000
    Re: Static caching property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-21 11:44 -0600
    Re: Static caching property Ethan Furman <ethan@stoneleaf.us> - 2016-03-21 10:45 -0700
    Re: Static caching property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-21 11:48 -0600
      Re: Static caching property Steven D'Aprano <steve@pearwood.info> - 2016-03-22 11:05 +1100
        Re: Static caching property Chris Angelico <rosuav@gmail.com> - 2016-03-22 11:15 +1100
        Re: Static caching property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-22 07:30 -0600

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