[Python-Dev] PEP 447 (type.__getdescriptor__)

Ronald Oussoren ronaldoussoren at mac.com
Wed Jul 22 19:24:27 CEST 2015

> On 22 Jul 2015, at 19:12, Steve Dower <Steve.Dower at microsoft.com> wrote:
> Terry Reedy wrote:
>> On 7/22/2015 3:25 AM, Ronald Oussoren wrote:
>>> Hi,
>>> Another summer with another EuroPython, which means its time again to
>>> try to revive PEP 447…
>>> I’ve just pushes a minor update to the PEP and would like to get some
>>> feedback on this, arguably fairly esoteric, PEP.
>> Yeh, a bit too esoteric for most of us to review. For instance, it is not
>> obvious to me, not familiar with internal details, after reading the intro, why
>> a custom __getattribute__ is not enough and why __getdescriptor__ would be
>> needed. If Guido does not want to review this, you need to find a PEP BDFL for
>> this.
>> There are two fairly obvious non-esoteric questions:
>> 1. How does this impact speed (updated section needed)?
> Agreed, this is important. But hopefully it's just a C indirection (or better yet, a null check) for objects that don't override __getdescriptor__.

Not a null check, but a check for a specific function pointer. That way you can be sure that super classes always have the slot which IMHO gives a nicer user experience.

>> 2. Is this useful, that you can think of, for anything other than connecting to
>> Objective C?
> There are other object models that would benefit from this, but I don't recall that we came up with uses other than "helps proxy to objects where listing all members eagerly is expensive and/or potentially incorrect". Maybe once you list all the operating systems that are now using dynamic object-oriented APIs rather than flat APIs (Windows, iOS, Android, ... others?) this is good enough?
> FWIW, I'm still +1 on this, pending performance testing.

The PEP on the website contains performance test data, but that’s out of data. I don’t think the implementation of attribute lookup has changed enough to really invalidate those test results, but I will rerun the tests once I’ve updated the patch because hunches don’t count when evaluating performance.


> Cheers,
> Steve
>> --
>> Terry Jan Reedy
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