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

[Martin Teichmann]

Hi Sylvain,

thanks for the example, it's a great example to illustrate PEP 487 and
its design decisions.

> What it does is setting some class attributes that are required for certain
> types of descriptors to be able to initialize themselves.
>
> class MetaHasTraits(MetaHasDescriptors):
>
>     """A metaclass for HasTraits."""
>     def setup_class(cls, classdict):
>         cls._trait_default_generators = {}
>         super(MetaHasTraits, cls).setup_class(classdict)

While in a metaclass solution this does need that the metaclass needs
to execute code on the first class it is used for, in a PEP 487
solution this is not the case. A PEP 487 class HasTraits (no Meta
here) would not have Traits-descriptors itself, the classes inheriting
from it would have traits to be initialized. The PEP 487 HasTraits
takes the role of the metaclass.

A completely different problem shows up here. In your example,
HasTraits needs to initialize things on the class before the
Descriptors are run. This is not possible with PEP 487, where the
descriptors are initialized before __init_subclass__ is even called.

There are two ways to mitigate that problem:

- the first initialized descriptor could do the necessary initialization, or
- the descriptors are initialized from within __init_subclass__

At first sight, the first option seems hackish and the second option
looks much saner. Nonetheless PEP 487 proposes the second solution.
The rationale behind that is that people tend to forget to call
super(), and suddenly descriptors don't work anymore.

I realized later there is another benefit to this: if the first
initialized descriptor is doing the class initialization, often
__init_subclass__ isn't needed at all anymore, which means that those
kind of descriptors can be used on any class, without the need to tell
users that they have to inherit from a certain base class for the
descriptors to work. Only if some finalizing code needs to run after
the last descriptor is initialized one needs to write an
__init_subclass__. This is unavoidable as the last descriptor doesn't
know that it is the last.

Greetings

Martin