code: def zero_nan(self, *cols):
          
          if len(cols) == 0:
              self.warning("Can not nan zero values if a column name "
                           "is not provided")
          df = self._zero_nan(*cols)
          if df is None:
              self.err("Can not fill zero values with nan")
              return
          self.df = df
docs: Converts zero values to nan values in selected columns
  
          :param \*cols: names of the colums
          :type \*cols: str, at least one
  
          :example: ``ds.zero_nan("mycol1", "mycol2")``