code: def is_callable_tag(tag):
      
      return (isinstance(tag, six.string_types) and
              tag.strip().startswith('{{') and
              tag.strip().endswith('}}'))
docs: Determine whether :tag: is a valid callable string tag.
  
      String is assumed to be valid callable if it starts with '{{'
      and ends with '}}'.
  
      :param tag: String name of tag.