code: def test_median_imputation_when_user_enters_single_variables(df_na):
      # set up trasnformer
      imputer = MeanMedianImputer(
          imputation_method="median", variables=["Age"]
      )
      X_transformed = imputer.fit_transform(df_na)
  
      # set up reference output
      X_reference = df_na.copy()
      X_reference["Age"] = X_reference["Age"].fillna(23.0)
  
      # test init params
      assert imputer.imputation_method == "median"
      assert imputer.variables == ["Age"]
  
      # test fit attributes
      assert imputer.n_features_in_ == 6
      assert imputer.imputer_dict_ == {"Age": 23.0}
  
      # test transform output
      assert X_transformed["Age"].isnull().sum() == 0
      pd.testing.assert_frame_equal(X_transformed, X_reference)
  
imports: import pandas
  import pytest
  from feature_engine.imputation import MeanMedianImputer
label_desc: Use image node_id 2 for calling a global function with example usage: test_median_imputation_when_user_enters_single_variables(df_na) without return types