import json
import pymongo
from bson import ObjectId
import datetime

class JSONEncoder(json.JSONEncoder):
    ‘‘‘处理ObjectId,该类型无法转为json‘‘‘
    def default(self, o):
        if isinstance(o, ObjectId):
            return str(o)
        if isinstance(o, datetime.datetime):
            return datetime.datetime.strftime(o,‘%Y-%m-%d %H:%M:%S‘)
        return json.JSONEncoder.default(self, o)


if __name__ == "__main__":
    mongodb = ...
    conn = mongodb["dbname"]
    coll = conn["tbname"]
    cols = []
    for col in coll.find():
        col = JSONEncoder().encode(col)
        cols.append(col)
    data = json.dumps(cols)