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] }
```
