Easy caching

[M.-A. Lemburg]

On 2008-11-13 09:03, Peter Otten wrote:
> jalanb3 wrote:
> 
>> Evening all,
>>
>> And thank you for your valuable reading skills.
>>
>> The following pattern turned up in coding tonight.
>> It works, but I'm suspicious, it just seems too easy.
>>
>> So any comments or constructive criticisms welcome ?
>>
>> *** Start doctest format ***
>>
>>>>> class Cacher:
>> ...     def __init__(self):
>> ...         self.value_in_database = 0
>> ...
>> ...     def very_slow_database_access(self):
>> ...         return self.value_in_database
>> ...
>> ...     @property
>> ...     def show(self):
>> ...         result = self.very_slow_database_access()
>> ...         self.show = result
>> ...         return result
>> ...
> 
> The above only works because properties don't work with classic class. If
> you make Cacher a newstyle class (have it inherit from object) you'll get
> an AttributeError. Therefore I think it would be cleaner to either 
> 
> use __getattr__() or
> 
> change the Cacher to something like
> 
> class Cacher(object):
>     def __init__(self):
>         self._show = None
>     # ...
>     
>     @property
>     def show(self):
>         result = self._show
>         if result is None:
>             result = self._show = self.very_slow_database_access()
>         return result

You should also consider not using a property for this at all.

Attribute access is supposed to be fast and, if at all, only cause
AttributeErrors.

Your self.very_slow_database_access() call can raise all sorts of
errors and likely take much too long for what a casual user would
expect from writing print (cache.show).

It's much better to write this as method which then uses an
instance attribute for caching the value read from the database.

-- 
Marc-Andre Lemburg
eGenix.com

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