am a newbie but i have a question here. You wanted State to behave like an interface right and throw an error when run() and next() are not defined, then assert should be

assert 0, "run not implemented"

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f  e  x

The correct exception to raise for a method that is not implemented by class is NotImplementedError, so this would become:

class State:
    def run(self):
        raise NotImplementedError('State.run')
    def next(self):
        raise NotImplementedError('State.next')