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.

def isbn(self, fmt: Optional[ISBNFormat] = None,
             locale: str = 'en') -> str:
        """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.
        """
        fmt_value = self._validate_enum(item=fmt, enum=ISBNFormat)
        mask = ISBN_MASKS[fmt_value].format(
            ISBN_GROUPS[locale])
        return self.random.custom_code(mask)