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

Larry Hastings larry at hastings.org
Tue Sep 12 03:38:47 EDT 2017

On 09/11/2017 07:22 PM, Guido van Rossum wrote:

I still don't follow. How does one use InstanceDescriptor?

If you write a class that inherits from InstanceDescriptor and supports the descriptor protocol, module objects will call the descriptor protocol functions when that object is accessed inside a module instance.

Example: if this is "module.py":

import collections.abc
class CustomInstanceProperty(collections.abc.InstanceDescriptor):
     def __get__(self, instance, owner):
         print("CustomInstanceProperty.__get__ called!")
         return 44

cip = CustomInstanceProperty()

If you then "import module" and reference "module.cip", it'll print "CustomInstanceProperty.get called!" and you'll get the value 44.
All three magic methods for descriptors work fine.

(BTW, I didn't know the best way to make this work. I wound up implementing InstanceDescriptor by copying-and-pasting PyBaseObject_Type, setting the special tp_flag, and ensuring the special flag gets propagated to subclasses in inherit_special(). If there's a better way to do it I'd happily improve the implementation.)

Is this still about modules, or is it a generalization of properties for non-module instances? (If it is specific to modules, why doesn't it have "module" in its name? Or if it's not, why is it in this discussion?)

It's a generalization of supporting descriptors for instances, but currently it's only supported by modules. It's opt-in and modules are the only class doing so.

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

https://github.com/larryhastings/cpython/tree/module-properties <https://github.com/larryhastings/cpython/tree/module-properties> I found that link in the PEP, but it's just a branch of a fork of cpython. It would be easier to review the prototype as a PR to upstream cpython.

Okay, I'll make a PR tomorrow and post a reply here (and update the PEP).

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