code: def _sanetize_fields(
      categorical_fields: Optional[Union[str, List[str]]],
      numerical_fields: Optional[Union[str, List[str]]],
  ):
      if categorical_fields is None and numerical_fields is None:
          raise RuntimeError(
              "Both `categorical_fields` and `numerical_fields` are None!"
          )
  
      categorical_fields = categorical_fields or []
      numerical_fields = numerical_fields or []
  
      if not isinstance(categorical_fields, list):
          categorical_fields = [categorical_fields]
  
      if not isinstance(numerical_fields, list):
          numerical_fields = [numerical_fields]
  
      return categorical_fields, numerical_fields
  
file_path: lightning-flash/src/flash/tabular/input.py
imports: from io import StringIO
  from typing import Any, Dict, List, Optional, Union
  import numpy
  from flash.core.data.io.input import DataKeys, Input, ServeInput
  from flash.core.utilities.imports import _PANDAS_AVAILABLE
  from flash.tabular.classification.utils import _compute_normalization, _generate_codes, _pre_transform, _to_cat_vars_numpy, _to_num_vars_numpy
label_desc: Use image node_id 1 for calling the TabularDataFrameInput obj's underlying member method code with example usage: obj._sanetize_fields(categorical_fields, numerical_fields) and returns: categorical_fields,  numerical_fields