NESTED JSON to CSV in Python

jsoncsv.py

import copy
import json

def flat_json(data, last_name = "", rows = [{}], keys = []):
    last_row = rows[len(rows)-1]  
    if type(data) is dict:
        lists = {}
        for key in data:    
            name = last_name + ("__" if last_name else "") + key
            if type(data[key]) is list:
                lists[key] = data
            else:
                value = flat_json(data[key], name, rows, keys)
                if value != None:
                    last_row[name] = value
                    if not name in keys:
                        keys.append(name)
        for key in lists:
            name = last_name + ("__" if last_name else "") + key
            flat_json(data[key], name, rows, keys)
    elif type(data) is list:
        if len(data) > 0:
            flat_json(data[0], last_name, rows, keys)
            for idx in range(1, len(data)):
                rows.append(copy.deepcopy(last_row))
                flat_json(data[idx], last_name, rows, keys)
    else:
        return data

def json_to_csv(data):
    out = [{}]
    keys = []
    flat_json(data, "", out, keys)
    out_str = ",".join(map(lambda x: '"'+x+'"',keys)) + "\n"
    for row in out:
        row_str = ""
        for key in keys:
            if key in row:
                row_str += ',"' + str(row[key]) + '"' 
            else:
                row_str += ',""'
        out_str += row_str[1:] + "\n"
    return out_str