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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def country_code(self, fmt: Optional[CountryCode] = CountryCode.A2) -> str: """Get a random code of country. Default format is :attr:`~enums.CountryCode.A2` (ISO 3166-1-alpha2), you can change it by passing parameter ``fmt`` with enum object :class:`~enums.CountryCode`. :param fmt: Enum object CountryCode. :return: Country code in selected format. :raises KeyError: if fmt is not supported. """

        key = self._validate_enum(fmt, CountryCode)
        return self.random.choice(COUNTRY_CODES[key])