code: def test_string_selection_mode(create_X_y):
      X, y = create_X_y
      ds = BaseDS(knn_classifier="knn")
      ds.fit(X, y)
      assert isinstance(ds.roc_algorithm_, KNeighborsClassifier)
  
imports: import unittest.mock
  from unittest.mock import Mock, MagicMock
  import numpy
  import pytest
  from sklearn.exceptions import NotFittedError
  from sklearn.neighbors import KNeighborsClassifier
  from deslib.base import BaseDS
  from deslib.util.dfp import frienemy_pruning_preprocessed
  from .conftest import create_base_classifier
label_desc: Use image node_id 12 for calling a global function with example usage: test_string_selection_mode(create_X_y) without return types