class_docstr: Print the evaluation result at each iteration.
  
  .. versionadded:: 1.3.0
  
  Parameters
  ----------
  
  rank :
      Which worker should be used for printing the result.
  period :
      How many epoches between printing.
  show_stdv :
      Used in cv to show standard deviation.  Users should not specify it.
class_docstr_tok: ["Print","the","evaluation","result","at","each","iteration",".","..","versionadded",":",":","1.3.0","Parameters","--","--","--","--","--","rank",":","Which","worker","should","be","used","for","printing","the","result",".","period",":","How","many","epoches","between","printing",".","show_stdv",":","Used","in","cv","to","show","standard","deviation",".","Users","should","not","specify","it","."]
code: def _fmt_metric(
      self, data: str, metric: str, score: float, std: Optional[float]
  ) -> str:
      if std is not None and self.show_stdv:
          msg = f"\t{data + '-' + metric}:{score:.5f}+{std:.5f}"
      else:
          msg = f"\t{data + '-' + metric}:{score:.5f}"
      return msg
  
imports: import collections
  import os
  import pickle
  from abc import ABC
  from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple, TypeVar, Union, cast
  import numpy
  from .None import collective
  from .core import Booster, DMatrix, XGBoostError, _parse_eval_str
label_desc: Use image node_id 2 for calling the EvaluationMonitor obj's underlying member method code with example usage: obj._fmt_metric(data, metric, score, std) and returns: msg