[Python-Dev] PEP 549: Instance Properties (aka: module properties)

Larry Hastings larry at hastings.org
Mon Sep 11 21:04:33 EDT 2017

On 09/11/2017 08:44 AM, Guido van Rossum wrote:
> I worry that in the end @property isn't general enough and the major 
> use cases end up still having to use __class__ assignment, and then 
> we'd have a fairly useless feature that we cant withdraw, ever.

What can I say--I don't have that worry ;-)

As previously mentioned in this thread, I counted up uses of property, 
__getattr__, and __getattribute__ in 3.7/Lib.  I grepped for the 
following strings, ignored pydoc_data/topics.py, and got these totals:

    "@property" 375 hits
    "def __getattr__" 28 hits
    "def __getattribute__(" 2 hits

@property seems pretty popular.

>>     Why is there no mechanism to add new descriptors that can work in
>>     this context?
>     I've updated the prototype to add one.  I added it as
>     "collections.abc.InstanceDescriptor"; that's a base class you can
>     inherit from, and then your descriptor will work in a module. 
>     Bikeshedding the name is fine.
> I don't understand the question, or the answer. (And finding the 
> prototype is taking longer than writing this email.)

Ronald was basically asking: what about user classes?  The first 
revision of the prototype didn't provide a way to write your own 
instance descriptors.  The only thing that could be a instance 
descriptor was property.  So, I updated the prototype and added 
collections.abc.InstanceDescriptor, a base class user classes can 
inherit from that lets them be instance descriptors.

The prototype is linked to from the PEP; for your convenience here's a link:


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