Neil Girdhar added the comment:

>From your comment:

>>> MyDerivedDynamic = new_class("MyDerivedDynamic", (MyClass,), 
>>> dict(metaclass=metaclass_callable))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/", line 57, in new_class
    return meta(name, bases, ns, **kwds)
  File "<stdin>", line 2, in metaclass_callable
TypeError: metaclass conflict: the metaclass of a derived class must be a 
(non-strict) subclass of the metaclasses of all its bases

This is in the wrong place.  It should be tripping the exception you defined in 
Lib/  (It will do that if you replace metaclass_callable with 


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