code: def set_input_by_number(self, number, value):
          
  
          sel = XpathSelector(self.form)
          elem = sel.select('.//input[@type="text"]')[number].node()
          return self.set_input(elem.get('name'), value)
docs: Set the value of form element by its number in the form
  
          :param number: number of element
          :param value: value which should be set to element