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

[Eric Snow]

On Tue, Mar 17, 2015 at 9:55 AM, Martin Teichmann
<lkb.teichmann at gmail.com> wrote:
> 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.

>From what I've read, PEP 487 is adding functionality to classes that
is already provided by metaclasses, but metaclasses still provide the
identical capability.  Basically, what is the difference between the
following?

  class Meta(type):
      def __init__(cls, ...):
          ...

  class X(metaclass=Meta):
      ...

and

  class Base:
      def __init_subclass__(cls, ...):
          ...

  class X(Base):
      ...

The only difference I see is relative to metaclass conflcts.  You
still have to define an extra type (Meta or Base) to get the effect.
Furthermore, there would now be second (but indistinct) way of
initializing a subclass, making the "meta" class situation even
murkier.

If the only benefit is the mitigation of metaclass conflicts then
perhaps such conflicts should be addressed directly and we should not
add __init_subclass__.  From other threads it sounds like we should be
able to solve metaclass conflicts one way or another.

Another thing I'm unclear on is what will happen if a subclass were to
define its own __init_subclass__?

-eric