Don't use six, just use zip

    kwargs['deterministic'] = True

    for epoch in range(epochs):

      iterators = [self.datasets[x].iterbatches(**kwargs) for x in key_order]

      for tup in six.moves.zip(*iterators):

      for tup in zip(*iterators):

        m_d = {key_order[i]: tup[i][0] for i in range(len(key_order))}

        yield m_d