Exception raised::

  Traceback (most recent call last):
    File "bug.py", line 5, in <module>
      class derived(type, Sized):
    File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/abc.py", line 85, in __new__
      for name in getattr(base, "__abstractmethods__", set()):
  TypeError: Error when calling the metaclass bases
      'getset_descriptor' object is not iterable

+- same traceback in 3.1
Since ABCmeta is not used (by name) its import is not needed.
I have no opinion on whether this should work.

If we create a new class, which is a metaclass, and also inherits an ABC, we create a new instance of ABCMeta.

When ABCMeta.__new__ creates the __abstractmethods__ attribute of the new class, it iterates over the __abstractmethods__ attribute of every base class (and checks every name in it, if it's abstract in the new class). One of the base classes of a metaclass is of course type. The type.__abstractmethods__ object is a getset descriptor (its __set__ switches some flags in tp_flags), which isn't iterable, so it raises the exception. 

Ideas for possible solutions:
1. In ABCMeta.__new__ special case type (do not check its __abstractmethods__attribute). 
2. In ABCMeta.__new__ check if the __abstractmethods__ attribute is in fact an iterable, before we try to iterate over it. (I don't think this would be a very good solution, because if a base class' __abstractmethods__ isn't iterable, it should be an error, except if the base class is type itself. So we should special case type anyway.)

I can't come up with a use case for this right now, but I don't see why it shouldn't work.

Does the "fix" for issue 10006 affect this?  (I imagine that question is why Antoine made Benjamin nosy on this issue).

You can now create metaclass abcs. However, having __abstractmethods__ does not prevent instance creation. This is a problem with a builtins, though.