PEP 487 introduced __init_subclass__ and __set_name__, and both of those were wins for the common cases of metaclass usage.

Unfortunately, the implementation of PEP 487 with regards to __init_subclass__ has made the writing of correct metaclasses significantly harder, if not impossible.

The cause is that when a metaclass calls type.__new__ to actually create the class, type.__new__ calls the __init_subclass__ methods of the new class' parents, passing it the newly created, but incomplete, class.  In code:

```
class Meta(type):
    #
    def __new__(mcls, name, bases, namespace, **kwds):
        # create new class, which will call __init_subclass__ and __set_name__
        new_class = type.__new__(mcls, name, bases, namespace, **kwds)
        # finish setting up class
        new_class.some_attr = 9
```

As you can deduce, when the parent __init_subclass__ is called with the new class, `some_attr` has not been added yet -- the new class is incomplete.

For Enum, this means that __init_subclass__ doesn't have access to the new Enum's members (they haven't beet added yet).

For ABC, this means that __init_subclass__ doesn't have access to __abstract_methods__ (it hasn't been created yet).

Because Enum is pure Python code I was able to work around it:
- remove new __init_subclass__ (if it exists)
- insert dummy class with a no-op __init_subclass__
- call type.__new__
- save any actual __init_subclass__
- add back any new __init_subclass__
- rewrite the new class' __bases__, removing the no-op dummy class
- finish creating the class
- call the parent __init_subclass__ with the now complete Enum class

I have not been able to work around the problem for ABC.

The solution would seem to be to move the calls to __init_subclass__ and __set_names__ to type.__init__.

My understanding of using keywords in class headers

  class MyClass(SomeBaseClass, setting='maybe'):
    ...

is that the keywords would get passed into the super classes `__init_subclass__`  (`SomeBaseClass` and `setting`).

However, in the cases of 

- test_descr.py
- test_py_compile.py
- typing.py

that wasn't happening -- until the initial patch of moving the calls from `type.__new__` to `type.__init__`.

An `__init__` has been added in those three locations to discard the keyword arguments being passed in.

Whoa, you start a discussion on python-dev and another on bpo? That sounds a bit hasty.

Guido, I just wanted to get it all in place while it was fresh in my mind.  Actual code tends to make a discussion easier.  I'll make sure and transcribe any relevant discussion from python-dev to here.

I see this as a backwards incompatible change and it's not worth doing for what seems to me a very minor benefit.

Nick Coghlan made the observation that `__set_name__` should be doing what is currently the after-new work.

Tracking in #42901.

Thanks, that's great! And thanks, Nick!