docstr: Get best_score.
docstr_tok: ["Get","best_score","."]
code: def best_score(self) -> float:
      """Get best_score."""
      return float(cast(float, self.cvfolds[0].bst.attr("best_score")))
  
imports: import copy
  import os
  import warnings
  from typing import Any, Dict, Iterable, List, Optional, Sequence, Tuple, Union, cast
  import numpy
  from ._typing import BoosterParam, Callable, FPreProcCallable
  from .callback import CallbackContainer, EarlyStopping, EvaluationMonitor, TrainingCallback
  from .compat import SKLEARN_INSTALLED, DataFrame, XGBStratifiedKFold
  from .core import Booster, DMatrix, Metric, Objective, XGBoostError, _deprecate_positional_args
label_desc: Use image node_id 10 for calling the _PackedBooster obj's underlying member method code with example usage: obj.best_score() and returns: float