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 bpo-42901.

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