This is my first post on python-dev and I hope that I am not breaking any rule.

I wanted to react on the discussion regarding PEP487.

This year, we have been working on a refactoring of the `traitlets` library, an implementation of the descriptor pattern that is used in Project Jupyter / IPython. The motivations for the refactoring was similar to those of this PEP: having a more generic metaclass allowing more flexibility in terms of types of descriptors, in order to avoid conflicts between meta classes.

We ended up with:

- A metaclass called MetaHasDescriptor

- A base class of meta MetaHasDescriptor named HasDescriptors

Usage:

class MyClass(HasDescriptors):

    attr = DesType()

DesType inherits from a base Descriptor type. The key is that their initialization is done in three stages

 - the main 

        DesType.__init__

 - the part of the initialization of DesType that depends on the definition of MyClass

        DesType.class_init(self, cls, name)

   which is called from MetaHasDescriptors.__new__

 - a method of DesType that depends on the definition of instances of MyClass

        DesType.instance_init(self, obj)

    which is called from HasDescriptors.__new__.

    instance_init, may make modifications on the HasDescriptors instance.

My understanding is that the proposed __set_name__ in PEP487 exactly corresponds to our class_init, although interestingly we often do much more in class_init than setting the name of the descriptor, such as setting a this_class attribute or calling class_init on contained descriptors. Therefore I do not think that the names __set_name__ or __set_owner__ are appropriate for this use case. 

In a way, the long-form explicit names for our class_init and instance_init methods would be something like __init_fom_owner_class__, and __touch_instance__.

PS: thanks to Neil Girdhar for the heads up on the traitlets repo.