[Python-Dev] Instance variable access and descriptors

Eyal Lotem eyal.lotem at gmail.com
Sun Jun 10 03:13:38 CEST 2007

I must be missing something, as I really see no reason to keep the
existing semantics other than backwards compatibility (which can be
achieved by introducing a __fastattr__ or such).

Can you explain under which situations or find any example situation
where the existing semantics are desirable?

As for the mro cache - thanks for pointing it out - I think it can
serve as a platform for another idea that in conjunction with psyco,
can possibly speed up CPython very significantly (will create a thread
about this soon).

Please note that speeding up the mro-lookup solves only half of the
problem (if it was solved - which it seems not to have been), the more
important half of the problem remains, allow me to emphasize:

ALL instance attribute accesses look up in both instance and class
dicts, when it could look just in the instance dict. This is made
worse by the fact that the class dict lookup is more expensive (with
or without the mro cache).
Some code that accesses a lot of instance attributes in an inner loop
can easily be sped up by a factor of 2 or more (depending on the size
of the mro).


On 6/10/07, Kevin Jacobs <jacobs at bioinformed.com> <bioinformed at gmail.com> wrote:
> I agree with Phillip with regard to the semantics.  They are semantically
> desirable.  However, there is a patch to add a mro cache to speed up these
> sorts of cases on the Python tracker, originally submitted by Armin Rigo.
> He saw ~20% speedups, others see less.  It is currently just sitting there
> with no apparent activity.  So if the overhead of mro lookups is that
> bothersome, it may be well worth your time to review the patch.
> URL:
> http://sourceforge.net/tracker/index.php?func=detail&aid=1700288&group_id=5470&atid=305470
> -Kevin
> On 6/9/07, 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.
> >
> >
> > >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.
> >
> > 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.
> >
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