code: def _get_na_values(col, na_values, na_fvalues, keep_default_na):
      
  
      if isinstance(na_values, dict):
          if col in na_values:
              return na_values[col], na_fvalues[col]
          else:
              if keep_default_na:
                  return _NA_VALUES, set()
  
              return set(), set()
      else:
          return na_values, na_fvalues
docs: Get the NaN values for a given column.
  
      Parameters
      ----------
      col : str
          The name of the column.
      na_values : array-like, dict
          The object listing the NaN values as strings.
      na_fvalues : array-like, dict
          The object listing the NaN values as floats.
      keep_default_na : bool
          If `na_values` is a dict, and the column is not mapped in the
          dictionary, whether to return the default NaN values or the empty set.
  
      Returns
      -------
      nan_tuple : A length-two tuple composed of
  
          1) na_values : the string NaN values for that column.
          2) na_fvalues : the float NaN values for that column.