Return comments for a case or variant.

        Args:
            case_id (str): id for a related case
            variant_id (Optional[str]): id for a related variant

def comments(self, case_id=None, variant_id=None, username=None):
        """Return comments for a case or variant.

        Args:
            case_id (str): id for a related case
            variant_id (Optional[str]): id for a related variant
        """
        logger.debug("Looking for comments")
        comment_objs = self.query(Comment)

        if case_id:
            comment_objs = comment_objs.filter_by(case_id=case_id)

        if variant_id:
            comment_objs = comment_objs.filter_by(variant_id=variant_id)
        elif case_id:
            comment_objs = comment_objs.filter_by(variant_id=None)

        return comment_objs