code: def test_drop_constant_features(df_constant_features):
      transformer = DropConstantFeatures(tol=1, variables=None)
      X = transformer.fit_transform(df_constant_features)
  
      # expected result
      df = pd.DataFrame(
          {
              "Name": ["tom", "nick", "krish", "jack"],
              "City": ["London", "Manchester", "Liverpool", "Bristol"],
              "Age": [20, 21, 19, 18],
              "Marks": [0.9, 0.8, 0.7, 0.6],
              "dob": pd.date_range("2020-02-24", periods=4, freq="T"),
              "quasi_feat_num": [1, 1, 1, 2],
              "quasi_feat_cat": ["a", "a", "a", "b"],
          }
      )
  
      # fit attribute
      assert transformer.features_to_drop_ == [
          "const_feat_num",
          "const_feat_cat",
      ]
  
      # transform output
      pd.testing.assert_frame_equal(X, df)
  
file_path: feature_engine/tests/test_selection/test_drop_constant_features.py
imports: import numpy
  import pandas
  import pytest
  from feature_engine.selection import DropConstantFeatures
label_desc: Use image node_id 2 for calling a global function with example usage: test_drop_constant_features(df_constant_features) without return types