How to use the iso3166.countries_by_alpha2 function in iso3166

def _validate_country_code(self):
        country_code = self.country_code
        try:
            iso3166.countries_by_alpha2[country_code]
        except KeyError:
            raise ValueError("Invalid country code '{}'".format(country_code))

from iso3166 import (
            countries_by_alpha2,
            countries_by_alpha3,
            Country as CountryType,
        )

        value = self._filter(value, Type(string_types + (CountryType,)))

        if self._has_errors:
            return None

        if isinstance(value, CountryType):
            return value

        search_functions = {
            2: countries_by_alpha2,
            3: countries_by_alpha3,
        }

        try:
            return search_functions[len(value)][value.upper()]
        except KeyError:
            return self._invalid_value(value, self.CODE_INVALID, exc_info=True)