[Python-Dev] Fwd: Instance variable access and descriptors

Gustavo Carneiro gjcarneiro at gmail.com
Sun Jun 10 12:27:16 CEST 2007

  I have to agree with you.  If removing support for
self.__dict__['propertyname'] (where propertyname is also the name of a
descriptor) is the price to pay for significant speedup, so be it.  People
doing that are asking for trouble anyway!

On 10/06/07, Eyal Lotem <eyal.lotem at gmail.com> wrote:
> On 6/10/07, Phillip J. Eby <pje at telecommunity.com> wrote:
> > At 12:23 AM 6/10/2007 +0300, Eyal Lotem wrote:
> > >A. It will break code that uses instance.__dict__['var'] directly,
> > >when 'var' exists as a property with a __set__ in the class. I believe
> > >this is not significant.
> > >B. It will simplify getattr's semantics. Python should _always_ give
> > >precedence to instance attributes over class ones, rather than have
> > >very weird special-cases (such as a property with a __set__).
> >
> > Actually, these are features that are both used and desirable; I've
> > been using them both since Python 2.2 (i.e., for many years
> > now).  I'm -1 on removing these features from any version of Python,
> even 3.0.
> It is the same feature, actually, two sides of the same coin.
> Why do you use self.__dict__['propertyname'] when you can use
> self._propertyname?
> Why even call the first form, which is both longer and causes
> performance problems "a feature"?
> > >C. It will greatly speed up instance variable access, especially when
> > >the class has a large mro.
> >
> > ...at the cost of slowing down access to properties and __slots__, by
> > adding an *extra* dictionary lookup there.
> It will slow down access to properties - but that slowdown is
> insignificant:
> A. The vast majority of lookups are *NOT* of properties. They are the
> rare case and should not be the focus of optimization.
> B. Property access involves calling Python functions - which is
> heavier than a single dict lookup.
> C. The dict lookup to find the property in the __mro__ can involve
> many dicts (so in those cases adding a single dict lookup is not
> heavy).
> > Note, by the way, that if you want to change attribute lookup
> > semantics, you can always override __getattribute__ and make it work
> > whatever way you like, without forcing everybody else to change *their*
> code.
> If I write my own __getattribute__ I lose the performance benefit that
> I am after.
> I do agree that code shouldn't be broken, that's why a transitional
> that requires using __fastlookup__ can be used (Unfortunately, from
> __future__ cannot be used as it is not local to a module, but to a
> class hierarchy - unless one imports a feature from __future__ into a
> class).
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Gustavo J. A. M. Carneiro
"The universe is always one step beyond logic." -- Frank Herbert
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