getting special from type, not instance (was Re: [Python-Dev] copy confusion)

[Armin Rigo]

Hi Guido,

On Wed, Jan 12, 2005 at 09:59:13AM -0800, Guido van Rossum wrote:
> The descriptor for __getattr__ and other special attributes could
> claim to be a "data descriptor"

This has the nice effect that x[y] and x.__getitem__(y) would again be
equivalent, which looks good.

On the other hand, I fear that if there is a standard "metamethod" decorator
(named after Phillip's one), it will be misused.  Reading the documentation
will probably leave most programmers with the feeling "it's something magical
to put on methods with __ in their names", and it won't be long before someone
notices that you can put this decorator everywhere in your classes (because it
won't break most programs) and gain a tiny performance improvement.

I guess that a name-based hack in type_new() to turn all __*__() methods into
data descriptors would be even more obscure?

Finally, I wonder if turning all methods whatsoever into data descriptors
(ouch! don't hit!) would be justifiable by the feeling that it's often bad
style and confusing to override a method in an instance (as opposed to
defining a method in an instance when there is none on the class).  
(Supporting this claim: Psyco does this simplifying hypothesis for performance
reasons and I didn't see yet a bug report for this.)

In all cases, I'm +1 on seeing built-in method objects (PyMethodDescr_Type)  
become data descriptors ("classy descriptors?" :-).


Armin