code: def do_display(self, arg):
          
          try:
              value = self._getval_or_undefined(arg)
          except:
              return
          self._get_display_list()[arg] = value
docs: display expression
  
          Add expression to the display list; expressions in this list
          are evaluated at each step, and printed every time its value
          changes.
  
          WARNING: since the expressions is evaluated multiple time, pay
          attention not to put expressions with side-effects in the
          display list.