code: def as_select(self, _items=None, **kwargs):
          
          attrs = self.extra.copy()
          attrs.update(kwargs)
          attrs['name'] = self.name
          if not self.optional:
              attrs['required'] = True
          html = [u'<select %s>' % get_html_attrs(attrs)]
          values = [self.to_string(**attrs)] or []
          items = _items or self.items
  
          for item in items:
              if isinstance(item, list):
                  html.extend(self._render_optgroup(item, values))
              else:
                  html.append(self._render_option(item, values))
          html.append(u'</select>')
  
          return Markup('\n'.join(html))
docs: Render the field as a `<select>` element.
  
          :param **kwargs:
              Named paremeters used to generate the HTML attributes of each item.
              It follows the same rules as `get_html_attrs`