Pythonic way to change one field in nested entry in MongoDB

Question

Since im fairly new to MongoDB (and databases in general), i wonder if this is the most pythonic way to modify one field in a nested entry. The field i want to change is stored in my collection "boards" like this:

_id: 270
checklist: {
    mechanical: {
        15: False
    }
}

All keys (checklist, mechanical and 15) are strings and refer to a bool value (False). The code on how i modify the data is the following:

# lock collection, define query
col = db["boards"]
query = {"_id": board_id}

# get data of one ID
document = col.find_one(query)
if document is None:
    print("Board id {0} invalid!".format(board_id))
    return

# modify document
document["checklist"][checklist][str(checklist_nr)] = set_to

# update whole document
col.update_one(query, {"$set": document})

As you can see, i always get the full document, modify it and write the full modified document again. Is there a way to just query this one field and modify it? I could not manage to do it, got errors like this, etc:

bson.errors.InvalidDocument: cannot encode object: {'15'}, of type: <class 'set'>