Converting between BSON and JSON

In many applications, records from MongoDB need to be serialized in JSON format. If your records have fields of type date, datetime, objectId, binary, code, etc. you will encounter TypeError: not JSON serializable exceptions when using json.dumps. This topic shows how to overcome this.

Using json_util

json_util provides two helper methods, dumps and loads, that wrap the native json methods and provide explicit BSON conversion to and from json.

Simple usage

from bson.json_util import loads, dumps
record = db.movies.find_one()
json_str = dumps(record)
record2 = loads(json_str)

if record is:

    "_id" : ObjectId("5692a15524de1e0ce2dfcfa3"), 
    "title" : "Toy Story 4", 
    "released" : ISODate("2010-06-18T04:00:00Z") 

then json_str becomes:

    "_id": {"$oid": "5692a15524de1e0ce2dfcfa3"},
    "title" : "Toy Story 4", 
    "released": {"$date": 1276833600000}


It is possible to customize the behavior of dumps via a JSONOptions object. Two sets of options are already available: DEFAULT_JSON_OPTIONS and STRICT_JSON_OPTIONS.

>>> bson.json_util.DEFAULT_JSON_OPTIONS
    JSONOptions(strict_number_long=False, datetime_representation=0,
     strict_uuid=False, document_class=dict, tz_aware=True, 
     uuid_representation=PYTHON_LEGACY, unicode_decode_error_handler='strict',
     tzinfo=<bson.tz_util.FixedOffset object at 0x7fc168a773d0>) 

To use different options, you can:

  1. modify the DEFAULT_JSON_OPTIONS object. In this case, the options will be used for all subsequent call to dumps:

     from bson.json_util import DEFAULT_JSON_OPTIONS
     DEFAULT_JSON_OPTIONS.datetime_representation = 2
  2. specify a JSONOptions in a call to dumps using the json_options parameter:

     # using strict
     dumps(record, json_options=bson.json_util.STRICT_JSON_OPTIONS)
     # using a custom set of options
     from bson.json_util import JSONOptions
     options = JSONOptions() # options is a copy of DEFAULT_JSON_OPTIONS
     dumps(record, json_options=options)

The parameters of JSONOptions are:

  • strict_number_long: If true, Int64 objects are encoded to MongoDB Extended JSON’s Strict mode type NumberLong, ie {"$numberLong": "<number>" }. Otherwise they will be encoded as an int. Defaults to False.
  • datetime_representation: The representation to use when encoding instances of datetime.datetime. 0 => {"$date": <dateAsMilliseconds>}, 1 => {"$date": {"$numberLong": "<dateAsMilliseconds>"}}, 2 => {"$date": "<ISO-8601>"}
  • strict_uuid: If true, uuid.UUID object are encoded to MongoDB Extended JSON’s Strict mode type Binary. Otherwise it will be encoded as {"$uuid": "<hex>" }. Defaults to False.
  • document_class: BSON documents returned by loads() will be decoded to an instance of this class. Must be a subclass of collections.MutableMapping. Defaults to dict.
  • uuid_representation: The BSON representation to use when encoding and decoding instances of uuid.UUID. Defaults to PYTHON_LEGACY.
  • tz_aware: If true, MongoDB Extended JSON’s Strict mode type Date will be decoded to timezone aware instances of datetime.datetime. Otherwise they will be naive. Defaults to True.
  • tzinfo: A datetime.tzinfo subclass that specifies the timezone from which datetime objects should be decoded. Defaults to utc.

Using the json module with custom handlers

If all you need is serializing mongo results into json, it is possible to use the json module, provided you define custom handlers to deal with non-serializable fields types. One advantage is that you have full power on how you encode specific fields, like the datetime representation.

Here is a handler which encodes dates using the iso representation and the id as an hexadecimal string:

import pymongo
import json 
import datetime
import bson.objectid

def my_handler(x):
    if isinstance(x, datetime.datetime):
        return x.isoformat()
    elif isinstance(x, bson.objectid.ObjectId):
        return str(x)
        raise TypeError(x)

db = pymongo.MongoClient().samples
record = db.movies.find_one()
# {u'_id': ObjectId('5692a15524de1e0ce2dfcfa3'), u'title': u'Toy Story 4',
#   u'released': datetime.datetime(2010, 6, 18, 4, 0),}

json_record = json.dumps(record, default=my_handler)
# '{"_id": "5692a15524de1e0ce2dfcfa3", "title": "Toy Story 4", 
#    "released": "2010-06-18T04:00:00"}'

Using python-bsonjs

python-bsonjs does not depend on PyMongo and can offer a nice performance improvement over json_util:

bsonjs is roughly 10-15x faster than PyMongo’s json_util at decoding BSON to JSON and encoding JSON to BSON.

Note that:

  • to use bsonjs effectively, it is recommended to work directly with RawBSONDocument
  • dates are encoded using the LEGACY representation, i.e. {"$date": <dateAsMilliseconds>}. There is currently no options to change that.


pip install python-bsonjs


To take full advantage of the bsonjs, configure the database to use the RawBSONDocument class. Then, use dumps to convert bson raw bytes to json and loads to convert json to bson raw bytes:

import pymongo
import bsonjs
from pymongo import MongoClient
from bson.raw_bson import RawBSONDocument

# configure mongo to use the RawBSONDocument representation
db = pymongo.MongoClient(document_class=RawBSONDocument).samples
# convert json to a bson record
json_record = '{"_id": "some id", "title": "Awesome Movie"}' 
raw_bson = bsonjs.loads(json_record)
bson_record = RawBSONDocument(raw_bson)
# insert the record
result = db.movies.insert_one(bson_record)

# find some record
bson_record2 = db.movies.find_one()
# convert the record to json
json_record2 = bsonjs.dumps(bson_record2.raw)