def _get_specification(self, spec):
        """
        Helper method to create a homogeneous representation of a specified getter or
        getter/setter pair.

        :param spec: Function specification 'getter', (getter,) or (getter,setter)
        :return:  Harmonized getter/setter specification, (getter, setter)
        """
        if spec is None or callable(spec) or isinstance(spec, string_types):
            spec = (spec,)
        if len(spec) == 1:
            spec = (spec[0], None)
        return spec