code: def set_format(self, column_or_columns, formatter):
          
          if inspect.isclass(formatter):
              formatter = formatter()
          if callable(formatter) and not hasattr(formatter, 'format_column'):
              formatter = _formats.FunctionFormatter(formatter)
          if not hasattr(formatter, 'format_column'):
              raise Exception('Expected Formatter or function: ' + str(formatter))
          for label in self._as_labels(column_or_columns):
              if formatter.converts_values:
                  self[label] = formatter.convert_column(self[label])
              self._formats[label] = formatter
          return self
docs: Set the format of a column.