Get a random prefix for the International System of Units.

        :param sign: Sing of number.
        :param symbol: Return symbol of prefix.
        :return: Prefix for SI.
        :raises NonEnumerableError: if sign is not supported.

        :Example:
            mega

def prefix(self, sign: Optional[PrefixSign] = None,
               symbol: bool = False) -> str:
        """Get a random prefix for the International System of Units.

        :param sign: Sing of number.
        :param symbol: Return symbol of prefix.
        :return: Prefix for SI.
        :raises NonEnumerableError: if sign is not supported.

        :Example:
            mega
        """
        prefixes = SI_PREFIXES_SYM if \
            symbol else SI_PREFIXES

        key = self._validate_enum(item=sign, enum=PrefixSign)
        return self.random.choice(prefixes[key])