code: def _transform_na_fill(self, X):
      for column, value in self._na_fill_params.items():
          ind = pd.isnull(X.loc[:, column])
          X.loc[ind, column] = value
      return X
  
file_path: mljar-supervised/supervised/preprocessing/preprocessing_missing.py
imports: import json
  import os
  import numpy
  import pandas
  from supervised.preprocessing.preprocessing_utils import PreprocessingUtils
label_desc: Use image node_id 6 for calling the PreprocessingMissingValues obj's underlying member method code with example usage: obj._transform_na_fill(X) and returns: X