docs: def detect_converters(pattern: str,
                        converter_dict: Dict[str, Callable],
                        default: Callable = str):
      """ detect pairs of varname and converter from pattern"""
      converters = {}
      for matched in VARS_PT.finditer(pattern):
          matchdict = matched.groupdict()
          varname = matchdict['varname']
          converter = matchdict['converter']
          converters[varname] = converter_dict.get(converter, default)
      return converters