Python 3 FAQ

What Python 3 versions are supported?

PyMongo supports Python 3.x where x >= 2.

Are there any PyMongo behavior changes with Python 3?

Only one intentional change. Instances of bytes are encoded as BSON type 5 (Binary data) with subtype 0. In Python 3 they are decoded back to bytes. In Python 2 they will be decoded to Binary with subtype 0.

For example, let’s insert a bytes instance using Python 3 then read it back. Notice the byte string is decoded back to bytes:

Python 3.2.5 (default, Feb 26 2014, 12:40:25)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pymongo
>>> c = pymongo.MongoClient()
>>> c.test.bintest.insert_one({'binary': b'this is a byte string'}).inserted_id
ObjectId('4f9086b1fba5222021000000')
>>> c.test.bintest.find_one()
{'binary': b'this is a byte string', '_id': ObjectId('4f9086b1fba5222021000000')}

Now retrieve the same document in Python 2. Notice the byte string is decoded to Binary:

Python 2.7.6 (default, Feb 26 2014, 10:36:22)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pymongo
>>> c = pymongo.MongoClient()
>>> c.test.bintest.find_one()
{u'binary': Binary('this is a byte string', 0), u'_id': ObjectId('4f9086b1fba5222021000000')}

Why can’t I share pickled ObjectIds between some versions of Python 2 and 3?

Instances of ObjectId pickled using Python 2 can always be unpickled using Python 3. Due to http://bugs.python.org/issue13505 you must use Python 3.2.3 or newer to pickle instances of ObjectId if you need to unpickle them in Python 2.

If you pickled an ObjectId using Python 2 and want to unpickle it using Python 3 you must pass encoding='latin-1' to pickle.loads:

Python 2.7.6 (default, Feb 26 2014, 10:36:22)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> from bson.objectid import ObjectId
>>> oid = ObjectId()
>>> oid
ObjectId('4f919ba2fba5225b84000000')
>>> pickle.dumps(oid)
'ccopy_reg\n_reconstructor\np0\n(cbson.objectid\...'

Python 3.2.5 (default, Feb 26 2014, 12:40:25)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> pickle.loads(b'ccopy_reg\n_reconstructor\np0\n(cbson.objectid\...', encoding='latin-1')
ObjectId('4f919ba2fba5225b84000000')

If you need to pickle ObjectIds using Python 3 and unpickle them using Python 2 you must use Python 3.2.3 or newer and protocol <= 2:

Python 3.2.3 (v3.2.3:3d0686d90f55, Apr 10 2012, 11:25:50)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> from bson.objectid import ObjectId
>>> oid = ObjectId()
>>> oid
ObjectId('4f96f20c430ee6bd06000000')
>>> pickle.dumps(oid, protocol=2)
b'\x80\x02cbson.objectid\nObjectId\nq\x00)\x81q\x01c_codecs\nencode\...'

Python 2.6.9 (unknown, Feb 26 2014, 12:39:10)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> pickle.loads('\x80\x02cbson.objectid\nObjectId\nq\x00)\x81q\x01c_codecs\nencode\...')
ObjectId('4f96f20c430ee6bd06000000')

Unfortunately this won’t work if you pickled the ObjectId using a Python 3 version older than 3.2.3:

Python 3.2.2 (default, Mar 21 2012, 14:32:23)
[GCC 4.5.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> from bson.objectid import ObjectId
>>> oid = ObjectId()
>>> pickle.dumps(oid, protocol=2)
b'\x80\x02cbson.objectid\nObjectId\nq\x00)\x81q\x01c__builtin__\nbytes\...'

Python 2.4.6 (#1, Apr 12 2012, 14:48:24)
[GCC 4.5.3] on linux3
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> pickle.loads('\x80\x02cbson.objectid\nObjectId\nq\x00)\x81q\x01c__builtin__\nbytes\...')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/pickle.py", line 1394, in loads
    return Unpickler(file).load()
  File "/usr/lib/python2.4/pickle.py", line 872, in load
    dispatch[key](self)
  File "/usr/lib/python2.4/pickle.py", line 1104, in load_global
    klass = self.find_class(module, name)
  File "/usr/lib/python2.4/pickle.py", line 1140, in find_class
    klass = getattr(mod, name)
  AttributeError: 'module' object has no attribute 'bytes'

Warning

Unpickling in Python 2.6 or 2.7 an ObjectId pickled in a Python 3 version older than 3.2.3 will seem to succeed but the resulting ObjectId instance will contain garbage data.

>>> pickle.loads('\x80\x02cbson.objectid\nObjectId\nq\x00)\x81q\x01c__builtin__\nbytes\...)
ObjectId('5b37392c203135302c203234362c2034352c203235312c203136352c2033342c203532...')