code: def fill_nan(self, val: str, *cols):
          
          df = self._fill_nan(val, *cols)
          if df is not None:
              self.df = df
          else:
              self.err("Can not fill nan values")
docs: Fill NaN values with new values in the main dataframe
  
          :param val: new value
          :type val: str
          :param \*cols: names of the colums
          :type \*cols: str, at least one
  
          :example: ``ds.fill_nan("new value", "mycol1", "mycol2")``