docs: def get_feature_type(feature_column):
      """
      For a given feature, determine if it is real, binary or constant.
      Here binary means that only two unique values occur in the feature.
  
      :param feature_column: The feature column
      :type feature_column: pandas.Series
      :return: 'constant', 'binary' or 'real'
      """
      n_unique_values = len(set(feature_column.values))
      if n_unique_values == 1:
          _logger.warning("[test_feature_significance] Feature {} is constant".format(feature_column.name))
          return 'constant'
      elif n_unique_values == 2:
          return 'binary'
      else:
          return 'real'