[Python-Dev] PEP487: Simpler customization of class creation

[Sylvain Corlay]

Hello,

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__.

Thanks,

Sylvain

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