Python 3 FAQ

Contents

• Python 3 FAQ
  • What Python 3 versions are supported?
  • Are there any PyMongo behavior changes with Python 3?
  • Why can’t I share pickled ObjectIds between some versions of Python 2 and 3?

What Python 3 versions are supported?

PyMongo supports CPython 3.3+ and PyPy3.

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 3.3.5 (default, Apr 29 2016, 11:04:32)
[GCC 4.9.3] on linux
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.

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 3.3.5 (default, Apr 29 2016, 11:04:32)
[GCC 4.9.3] on linux
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 protocol <= 2:

Python 3.3.5 (default, Apr 29 2016, 11:04:32)
[GCC 4.9.3] on linux
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')