[Python-ideas] Simpler Customization of Class Creation, next round

[Martin Teichmann]

Hello everybody,

recently I posted PEP 487, a simpler customization of class creation.
For newcomers: I propose the introduction of a __init_subclass__ classmethod
which initializes subclasses of a class, simplifying what metaclasses can
already do.

It took me a while to digest all the ideas from the list here, but well, we're
not in a hurry. So, I updated PEP 487, and pushed the changes to github
at https://github.com/tecki/peps/commits/master

I applied the following changes:

PEP 487 contained the possibility to set a namespace for subclasses.
The most important usecase for this feature would be to have an
OrderedDict as the class definition namespace. As Eric Snow pointed
out, that will be soon standard anyways, so I took out this feature from
the PEP. The implementation on PyPI now just uses an OrderedDict
as a namespace, anticipating the expected changes to CPython.

I also did some reading on possible usecases for PEP 487, so that it
actually may be used by someone. Some Traits-like thing is a standard
usecase, so I looked especially at IPython's traitlets, which are a simple
example of that usecase.

Currently traitlets use both __new__ and __init__ of a metaclass. So I
tried to also introduce a __new_subclass__ the same way I introduced
__init_subclass__. This turned out much harder than I thought, actually
impossible, because it is type.__new__ that sets the method resolution
order, so making super() work in a __new_subclass__ hook is a
chicken-egg problem: we need the MRO to find the next base class to
call, but the most basic base class is the one creating the MRO. Nick,
how did you solve that problem in PEP 422?

Anyhow, I think that traitlets can also be written just using
__init_subclass__. There is just this weird hint in the docs that you should
use __new__ for metaclasses, not __init__, a hint I never understood
as the reasons when to use __new__ or __init__ are precisely the same
for normal classes and metaclasses. So I think we don't miss out much
when not having __new_subclass__.

I also updated the implementation of PEP 487, it's still at
https://pypi.python.org/pypi/metaclass

Greetings

Martin