data in "test":

```
{ "_id" : ObjectId("55ddabe6855a0a250877cc92"), "id" : 2, "value" : { "20140501" : 10, "20140502" : 1, "20140503" : 2 }}

```

I can update in mongodb by this:

```
db.test.update({id:2}, {$set:{'value.20140501':12}})

```

But when I use python, I can't be sure this key('20140501').I want to use variable to replace '20140501'.

```
client = pymongo.MongoClient("localhost", 27017)
db = client.test
table = db.test
date = '20140501'
table.find_one_and_update({'id':2}, {'$set': {"value.date":23423423}})

```

This is just not right.How to use variable in mongodb with python?