Neil Girdhar added the comment:

"As a result of this, *both* implementations include a conditional check for a 
more derived metaclass in their namespace preparation logic, as well as an 
unconditional call to that metaclass derivation logic from type_new if the 
calculated metaclass is either type itself, or a subclass that calls up to 
super().__new__."

I don't see why that's necessary.  Either you should have the check in one 
place, or else have two equivalent checks.  Right now, the Python library is 
confusingly checking a subset of cases (when isinstance(meta, type)).  I 
suggest that you have the Python library check the entire set of cases so that 
the raised exception shows up in the right place.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue28437>
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