Flask Request Parsing for list of list objects

Question

I have a JSON data in form

---

{"a": 
    [
        { 
            "1": [ { "A" : "A", "B" : "B" }, {"A" : "C", "B" : "D" } ],
            "2": { "E" : "E", "D" : "D"}
        }
    ]
}

Now, I want to validate this type of data. What I have done is

parser = reqparse.RequestParser()
parser.add_arguement("a",
    type=dict,
    action="append")
args = parser.parse(args)

But I have no idea how to parse objects inside the list.