def _id(self, type):
    if (type == 'single'):
        return self._identifier(Identifier.ISBN, self.SINGLE_ISBN)
    else:
        return self._identifier(Identifier.ISBN, self.MULTI_ISBN)
def _id(self, type):
    if (type == 'single'):
        return self._identifier(Identifier.ISBN, self.SINGLE_ISBN)
    else:
        return self._identifier(Identifier.ISBN, self.MULTI_ISBN)