fields.py

class CountryCodeField(serializers.Field):
    codes = {'US': 45, 'AU': 12}

    def to_internal_value(self, value):
        # make sure it is a string
        if type(value) is not str:
            raise ValidationError('`value` must be a string, you sent type %s' % type(value))

        # check length
        if len(value) != 2:
            raise ValidationError('value must be a 2 character ISO code, you sent %s' % value)
        
        try:
            return self.codes[value.upper()]
        except KeyError:
            raise ValidationError('Not a valid country code')
    
    def to_representation(self, value):
        inverse_lookup = dict([(numeric_id, code) for (code, numeric_id) in self.codes.items()])
        return inverse_lookup[value]