properties and get/set methods

[Mike C. Fletcher]

Terry Reedy wrote:

>"Mike C. Fletcher" <mcfletch at rogers.com> wrote in message
>news:mailman.1049604615.22077.python-list at python.org...
>  
>
>>In these cases, you're using properties to simplify the modelling of
>>domain objects, or to provide some *generic* functionality over and
>>above a regular value get/set.  You don't want to have to specify
>>    
>>
>that
>  
>
>>functionality for every property, you just want to say "this is one
>>    
>>
>of
>  
>
>>_those_ properties".  The code bloat (not to mention mindless
>>duplication of code) when you get up to 15 or 20 properties for an
>>object would be ridiculous.
>>    
>>
>
>Perhaps what you want are __get/setattr__ methods that check if the
>attribute is one of 'those-properties' (in a set/dict lookup) and if
>so, does the generically different thing.  My impression is that
>properties are meant for doing attribute specific customization
>without loading up/slowing down __get/setattr__ with a long list of
>if/elif blocks.
>
>Terry
>  
>

The annoyance there is that you then have to have every object aware of 
every property-type you want to support.  You either wind up with a 
"super property-holding class" from which every object must be derived, 
and which is aware of all of the property types, or you do holder-class 
mix-ins, or you have some sort of plug-in system to lookup the 
appropriate function based on the property type... sounds a lot like 
what you get with having the property's type (i.e. class) store the 
appropriate method to begin with.

When you get dozens of property-types, and individual properties need to 
store data about themselves (e.g. their constraints, their data-types) 
which requires another out-of-band storage mechanism if you're not 
storing them in the properties themselves, the use of an external 
__getattribute__/__setattr__ mechanism starts to fall down pretty fast. 
Shrug :) .

However, it works very well for such things as the original poster was 
describing (i.e. a single (or small number of) generic thing(s) to be 
done for a large fraction of properties of an object).  You can see a 
nice example in the Zope3 persistence module.

Enjoy all,
Mike

_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/