Re: Static caching property

From	Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups	comp.lang.python
Subject	Re: Static caching property
Date	2016-03-22 07:30 -0600
Message-ID	<mailman.11.1458653480.2244.python-list@python.org> (permalink)
References	(2 earlier) <56f0230e$0$1616$c3e8da3$5496439d@news.astraweb.com> <e043630368254fdeab16f0967df6662f@activenetwerx.com> <CAPTjJmqvu4qMpcbR-g5QZG6neuJVcHrKLYLDFe5QC_QBQbd2-g@mail.gmail.com> <mailman.453.1458582555.12893.python-list@python.org> <56f08c59$0$1590$c3e8da3$5496439d@news.astraweb.com>

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On Mon, Mar 21, 2016 at 6:05 PM, Steven D'Aprano <steve@pearwood.info> wrote:
> On Tue, 22 Mar 2016 04:48 am, Ian Kelly wrote:
>
>> You don't actually need a metaclass for this:
>>
>>>>> class Desc:
>> ...     def __get__(self, obj, type=None):
>> ...         if not type._cached_value:
>> ...             type._cached_value = compute_value(type)
>> ...         return type._cached_value
>
>
> This won't quite work. What if the cached value happens to be falsey, yet
> still expensive to compute? You should compare it to a known sentinel which
> the expensive function will never return (possibly None). Or use a hasattr
> test: if not hasattr(type, '_cached_value').

Sure. This was just a quick-and-dirty demonstration that I threw
together in 30 seconds.

> Also, you don't need the default type=None. The descriptor protocol should
> never call __get__ without supplying the type. The obj may be None, but I
> don't believe there are any circumstances where type will be None.

Why do the examples in the Python docs use type=None? I literally just
copy-pasted the method signature from there, since I can never
remember which argument comes first.

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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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