code: def print_evaluation(period=1, show_stdv=True):
      
      def _callback(env):
          if period > 0 and env.evaluation_result_list and (env.iteration + 1) % period == 0:
              result = '\t'.join([_format_eval_result(x, show_stdv) for x in env.evaluation_result_list])
              print('[%d]\t%s' % (env.iteration + 1, result))
      _callback.order = 10
      return _callback
docs: Create a callback that prints the evaluation results.
  
      Parameters
      ----------
      period : int, optional (default=1)
          The period to print the evaluation results.
      show_stdv : bool, optional (default=True)
          Whether to show stdv (if provided).
  
      Returns
      -------
      callback : function
          The callback that prints the evaluation results every ``period`` iteration(s).