code: def sentinel_checker():
      """Build :func:`callbacks.bases.lambda_callback` to compare the current `CVExperiment` dataset
      values with the expected values of the dataset (train, validation, and holdout) sentinels
  
      Returns
      -------
      LambdaCallback
          Result of :func:`callbacks.bases.lambda_callback` to check `DatasetSentinel` values
      """
  
      def on_run_start(data_train, data_oof, data_holdout):
          #################### Check Train Sentinels ####################
          assert data_train.input.T.fold.equals(
              G.Env.train_input.retrieve_by_sentinel()
          )
          assert data_train.target.T.fold.equals(
              G.Env.train_target.retrieve_by_sentinel()
          )
  
          #################### Check Validation Sentinels ####################
          assert data_oof.input.T.fold.equals(
              G.Env.validation_input.retrieve_by_sentinel()
          )
          assert data_oof.target.T.fold.equals(
              G.Env.validation_target.retrieve_by_sentinel()
          )
  
          #################### Check Holdout Sentinels ####################
          assert data_holdout.input.T.fold.equals(
              G.Env.holdout_input.retrieve_by_sentinel()
          )
          assert data_holdout.target.T.fold.equals(
              G.Env.holdout_target.retrieve_by_sentinel()
          )
  
      return lambda_callback(on_run_start=on_run_start)
  
file_path: hyperparameter_hunter/tests/test_sentinels.py
imports: from hyperparameter_hunter import Environment, CVExperiment, Real, Integer, Categorical, GBRT
  from hyperparameter_hunter import lambda_callback
  from hyperparameter_hunter.sentinels import DatasetSentinel
  from hyperparameter_hunter.settings import G
  from hyperparameter_hunter.utils.learning_utils import get_breast_cancer_data
  import pytest
  import sys
label_desc: Use image node_id 2 for calling a global function with example usage: sentinel_checker() and returns: lambda_callback