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Since inheritance is more commonplace and more easily understood than __metaclass__, the abc module would benefit from a simple helper class:
 class ABC(metaclass=ABCMeta):
 pass
From a user's point-of-view, writing an abstract base call becomes simpler and clearer:
 from abc import ABC, abstractmethod
 class Vector(ABC):
 @abstractmethod
 def __iter__(self):
 pass
 def dot(self, other):
 'Compute a dot product'
 return sum(map(operator.mul, self, other))
Notice that this class seems less mysterious because it inherits from ABC rather than using __metaclass__=ABCMeta.
Also note, it has become a reasonably common practice for metaclass writers to put the __metaclass__ assignment in a class and have it get inherited rather than requiring users do the metaclass assignment themselves.