code: def test_fillna():
      X = np.array([["a", np.nan, "c"], [np.nan, 2, 3]])
      selector = preprocessing.ImputeNull("a")
      X_t = selector.fit_transform(X)
      assert X_t.shape == X.shape
  
file_path: sk-dist/skdist/tests/test_preprocessing.py
label_desc: Use image node_id 5 for calling a global function with example usage: test_fillna() without return types