def __call__(self, doc, context=None):
        items = _evaluate_items_expression(self._items_expression, doc, context)
        #  all items should be iterable, if not return empty list
        for item in items:
            if not isinstance(item, list):
                return []
        try:
            return(list(itertools.chain(*items)))
        except __HOLE__:
            return []
dataset/ETHPy150Open dimagi/commcare-hq/corehq/apps/userreports/expressions/list_specs.py/FlattenExpressionSpec.__call__