def _normalize_group_attrs(self, attrs):
        """Normalize the attributes used to set groups
        If it's a list of one element, it just become this
        element.
        It raises an error if the attribute doesn't exist
        or if it's multivaluated.
        """
        for key in self.group_attrs_keys:
            if key not in attrs:
                raise MissingGroupAttr(key)
            if type(attrs[key]) is list and len(attrs[key]) == 1:
                attrs[key] = attrs[key][0]
            if type(attrs[key]) is list and len(attrs[key]) != 1:
                raise MultivaluedGroupAttr(key)