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

Guido van Rossum gvanrossum at gmail.com
Thu Jan 13 18:02:10 CET 2005

> 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 typenew() to turn all *() methods into data descriptors would be even more obscure?

To the contary, I just realized in this would in fact be the right approach. In particular, any descriptor named * would be considered a "data descriptor". Non-descriptors with such names can still be overridden in the instance dict (I believe this is used by Zope).

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.)

Alas, it's a documented feature that you can override a (regular) method by placing an appropriate callable in the instance dict.

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

Let's do override descriptors.

And please, someone fix copy.py in 2.3 and 2.4.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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