Summary:
Any class that derives from the ABCMeta class doesn't support keyword variable arguments as defined here :https://www.python.org/dev/peps/pep-3115/. 


Expected:
If i define a simple class that derives from ABCMeta that has a kwarg the class should be created (see below

from collections import UserDict
class MyDict(UserDict, bar='baz'):
    pass
dictionary = MyDict()  # Expect this to create a new instance of MyDict.


Actual:

from collections import UserDict
class MyDict(UserDict, bar='baz'):
    pass
dictionary = MyDict()  # This call fails because UserDict inherits from ABCMeta

Traceback (most recent call last):
  File "abc_meta.py", line 4, in <module>
    class MyDict(UserDict, bar='baz'):

ABCMeta does not support arbitrary keyword arguments, that is correct.  If you want keyword arguments to be handled, you need to write your own metaclass that does so.

(I'm pretty sure I'm reading the PEP correctly...if not I'm sure one of the other core devs will re-open this ;)

Hmm Ok. You are right i can do the following:

from collections import UserDict
from abc import ABCMeta


class MetaMyDict(ABCMeta):

    @classmethod
    def __prepare__(cls, name, bases, **kwargs):
        return {}

    def __new__(mcls, name, bases, namespace, **kwds):
        return super().__new__(mcls, name, bases, namespace)

    def __init__(cls, name, bases, namespace, **kargs):
        return super().__init__(name, bases, namespace)

class MyDict(UserDict, metaclass=MetaMyDict, bar='baz'):
    pass

dictionary = MyDict()

But I guess i would have expected a core lib library to be consistent with the data model https://docs.python.org/3.4/reference/datamodel.html#preparing-the-class-namespace. As it stands an end user can't get a subclass of ABCMeta to work with the same **kwargs interface without creating a custom metaclass that strips it out before passing to ABCMeta. 

Wouldn't it be much easier and technically correct for the core ABCMeta library to adopt the same interface contract for class creation introduced in python3?

> Wouldn't it be much easier and technically correct for the core ABCMeta library to adopt the same interface contract for class creation introduced in python3?

No, it would not be technically correct.  For the same reason, object.__init__ does not accept any parameters.

Yes, this is exactly analogous to object.__init__ not accepting arguments, but subclasses being free to do so.  ABCMeta *does* adopt the contract.  keyword arguments to the class constructor are not accepted unless you write a meta class that accepts them.  Period.  (That is, "class Foo(bar='baz'): pass" fails.)

Ahhh i see now. 

Even the simple case "class Foo(bar='baz'): pass" fails.

I misunderstood the documentation then. I thought that python 3 introduced a new interface for all classes when it actually just introduced the option to add keyword arguments to your own metaclasses but didn't alter the base level class interface.

I wanted a bit more background around the object.__init__ and found this ticket: http://bugs.python.org/issue1683368. While this is probably not the only place where this comes up I see the tact that the python core team took and understand your reference more completely now.

I thank both of you for your help and quick responses.