class_docstr: Test class for preprocessing tests.
docstr: Create necessary directories for testing.
docstr_tok: ["Create","necessary","directories","for","testing","."]
code: def setUpClass(cls):
      """Create necessary directories for testing."""
      for dir_path in [train_dir, test_dir, output_dir]:
          dir_path.mkdir(exist_ok=True)
  
imports: import json
  import os
  import re
  import unittest
  import numpy
  import scipy.sparse
  from numpy.testing import assert_almost_equal, assert_array_equal
  from sklearn.datasets import make_classification
  from sklearn.feature_extraction import FeatureHasher
  from skll.data import FeatureSet, NDJWriter
  from skll.experiments import run_configuration
  from skll.learner import Learner
  from skll.learner.utils import SelectByMinCount
  from skll.utils.testing import config_dir, fill_in_config_paths, output_dir, test_dir, train_dir, unlink
label_desc: Use image node_id 1 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.setUpClass(cls) without return types