Python 3 FAQ
============

.. contents::

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 :class:`bytes`
are encoded as BSON type 5 (Binary data) with subtype 0.
In Python 3 they are decoded back to :class:`bytes`. In
Python 2 they will be decoded to :class:`~bson.binary.Binary`
with subtype 0.

For example, let's insert a :class:`bytes` instance using Python 3 then
read it back. Notice the byte string is decoded back to :class:`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 :class:`~bson.binary.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 :class:`~bson.objectid.ObjectId` pickled using Python 2
can always be unpickled using Python 3. Due to
`http://bugs.python.org/issue13505 <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...')