Fixed error on Python 2

        # Perform optimization.

        for epoch in range(self.optimization_epochs):

          for batch in self._iter_batches(rollouts):

          if self.batch_size == 0:

            batches = rollouts

          else:

            batches = self._iter_batches(rollouts)

          for batch in batches:

            initial_rnn_states, state_arrays, discounted_rewards, actions_matrix, action_prob, advantages = batch

            # Build the feed dict and run the optimizer.

  def _iter_batches(self, rollouts):

    """Given a set of rollouts, merge them into batches for optimization."""

    if self.batch_size == 0:

      return rollouts

    # Merge all the rollouts into a single set of arrays.