code: def test_mahalanobis_metric():
      X, y = make_classification()
      ds_test = BaseDS(knn_metric="mahalanobis")
      ds_test._set_region_of_competence_algorithm(X)
      assert ds_test.roc_algorithm_.metric == "mahalanobis"
      assert np.array_equal(
          ds_test.roc_algorithm_.metric_params["VI"], np.cov(X)
      )
  
imports: import numpy
  import pytest
  from sklearn.datasets import make_classification
  from sklearn.ensemble import RandomForestClassifier
  from sklearn.model_selection import train_test_split
  from sklearn.preprocessing import StandardScaler
  from deslib.base import BaseDS
  from deslib.dcs.a_posteriori import APosteriori
  from deslib.dcs.mcb import MCB
  from deslib.dcs.ola import OLA
  from deslib.dcs.rank import Rank
  from deslib.des import DESKL
  from deslib.des.des_knn import DESKNN
  from deslib.des.des_p import DESP
  from deslib.des.knora_e import KNORAE
  from deslib.des.knora_u import KNORAU
  from deslib.des.meta_des import METADES
label_desc: Use image node_id 4 for calling a global function with example usage: test_mahalanobis_metric() without return types