code: def isbn(self, fmt: Optional[ISBNFormat] = None,
               locale: str = 'en') -> str:
          
          fmt_value = self._validate_enum(item=fmt, enum=ISBNFormat)
          mask = ISBN_MASKS[fmt_value].format(
              ISBN_GROUPS[locale])
          return self.random.custom_code(mask)
docs: Generate ISBN for current locale.
  
          To change ISBN format, pass parameter ``fmt`` with needed value of
          the enum object :class:`~mimesis.enums.ISBNFormat`
  
          :param fmt: ISBN format.
          :param locale: Locale code.
          :return: ISBN.
          :raises NonEnumerableError: if fmt is not enum ISBNFormat.