如果某个字段没有输出，那么用`$type:2` ``` if username == []: username = {"$type" : 2} else : username = {"$in":username} ``` directory: ['/remote/.*/IMPL/.*'], ``` if directory == [] : directory = {"$type" : 2} else : directory = {"$in": [re.compile("^"+d+"$") for d in directory] } ```