path: root/Doc/library/pickle.rst

:mod:`pickle` --- Python object serialization
=============================================

.. index::
   single: persistence
   pair: persistent; objects
   pair: serializing; objects
   pair: marshalling; objects
   pair: flattening; objects
   pair: pickling; objects

.. module:: pickle
   :synopsis: Convert Python objects to streams of bytes and back.
.. sectionauthor:: Jim Kerr <jbkerr@sr.hp.com>.
.. sectionauthor:: Barry Warsaw <barry@zope.com>

The :mod:`pickle` module implements a fundamental, but powerful algorithm for
serializing and de-serializing a Python object structure.  "Pickling" is the
process whereby a Python object hierarchy is converted into a byte stream, and
"unpickling" is the inverse operation, whereby a byte stream is converted back
into an object hierarchy.  Pickling (and unpickling) is alternatively known as
"serialization", "marshalling," [#]_ or "flattening", however, to avoid
confusion, the terms used here are "pickling" and "unpickling"..


Relationship to other Python modules
------------------------------------

The :mod:`pickle` module has an transparent optimizer (:mod:`_pickle`) written
in C. It is used whenever available. Otherwise the pure Python implementation is
used.

Python has a more primitive serialization module called :mod:`marshal`, but in
general :mod:`pickle` should always be the preferred way to serialize Python
objects.  :mod:`marshal` exists primarily to support Python's :file:`.pyc`
files.

The :mod:`pickle` module differs from :mod:`marshal` several significant ways:

* The :mod:`pickle` module keeps track of the objects it has already serialized,
  so that later references to the same object won't be serialized again.
  :mod:`marshal` doesn't do this.

  This has implications both for recursive objects and object sharing.  Recursive
  objects are objects that contain references to themselves.  These are not
  handled by marshal, and in fact, attempting to marshal recursive objects will
  crash your Python interpreter.  Object sharing happens when there are multiple
  references to the same object in different places in the object hierarchy being
  serialized.  :mod:`pickle` stores such objects only once, and ensures that all
  other references point to the master copy.  Shared objects remain shared, which
  can be very important for mutable objects.

* :mod:`marshal` cannot be used to serialize user-defined classes and their
  instances.  :mod:`pickle` can save and restore class instances transparently,
  however the class definition must be importable and live in the same module as
  when the object was stored.

* The :mod:`marshal` serialization format is not guaranteed to be portable
  across Python versions.  Because its primary job in life is to support
  :file:`.pyc` files, the Python implementers reserve the right to change the
  serialization format in non-backwards compatible ways should the need arise.
  The :mod:`pickle` serialization format is guaranteed to be backwards compatible
  across Python releases.

.. warning::

   The :mod:`pickle` module is not intended to be secure against erroneous or
   maliciously constructed data.  Never unpickle data received from an untrusted or
   unauthenticated source.

Note that serialization is a more primitive notion than persistence; although
:mod:`pickle` reads and writes file objects, it does not handle the issue of
naming persistent objects, nor the (even more complicated) issue of concurrent
access to persistent objects.  The :mod:`pickle` module can transform a complex
object into a byte stream and it can transform the byte stream into an object
with the same internal structure.  Perhaps the most obvious thing to do with
these byte streams is to write them onto a file, but it is also conceivable to
send them across a network or store them in a database.  The module
:mod:`shelve` provides a simple interface to pickle and unpickle objects on
DBM-style database files.


Data stream format
------------------

.. index::
   single: XDR
   single: External Data Representation

The data format used by :mod:`pickle` is Python-specific.  This has the
advantage that there are no restrictions imposed by external standards such as
XDR (which can't represent pointer sharing); however it means that non-Python
programs may not be able to reconstruct pickled Python objects.

By default, the :mod:`pickle` data format uses a compact binary representation.
The module :mod:`pickletools` contains tools for analyzing data streams
generated by :mod:`pickle`.

There are currently 4 different protocols which can be used for pickling.

* Protocol version 0 is the original ASCII protocol and is backwards compatible
  with earlier versions of Python.

* Protocol version 1 is the old binary format which is also compatible with
  earlier versions of Python.

* Protocol version 2 was introduced in Python 2.3.  It provides much more
  efficient pickling of :term:`new-style class`\es.

* Protocol version 3 was added in Python 3.0.  It has explicit support for
  bytes and cannot be unpickled by Python 2.x pickle modules. This is
  the current recommended protocol, use it whenever it is possible.

Refer to :pep:`307` for more information.

If a *protocol* is not specified, protocol 3 is used.  If *protocol* is
specified as a negative value or :const:`HIGHEST_PROTOCOL`, the highest
protocol version available will be used.


Usage
-----

To serialize an object hierarchy, you first create a pickler, then you call the
pickler's :meth:`dump` method.  To de-serialize a data stream, you first create
an unpickler, then you call the unpickler's :meth:`load` method.  The
:mod:`pickle` module provides the following constant:


.. data:: HIGHEST_PROTOCOL

   The highest protocol version available.  This value can be passed as a
   *protocol* value.

.. note::

   Be sure to always open pickle files created with protocols >= 1 in binary mode.
   For the old ASCII-based pickle protocol 0 you can use either text mode or binary
   mode as long as you stay consistent.

   A pickle file written with protocol 0 in binary mode will contain lone linefeeds
   as line terminators and therefore will look "funny" when viewed in Notepad or
   other editors which do not support this format.

.. data:: DEFAULT_PROTOCOL

   The default protocol used for pickling.  May be less than HIGHEST_PROTOCOL.
   Currently the default protocol is 3; a backward-incompatible protocol
   designed for Python 3.0.


The :mod:`pickle` module provides the following functions to make the pickling
process more convenient:

.. function:: dump(obj, file[, protocol])

   Write a pickled representation of *obj* to the open file object *file*.  This
   is equivalent to ``Pickler(file, protocol).dump(obj)``.

   The optional *protocol* argument tells the pickler to use the given protocol;
   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
   backward-incompatible protocol designed for Python 3.0.

   Specifying a negative protocol version selects the highest protocol version
   supported.  The higher the protocol used, the more recent the version of
   Python needed to read the pickle produced.

   The *file* argument must have a write() method that accepts a single bytes
   argument. It can thus be a file object opened for binary writing, a
   io.BytesIO instance, or any other custom object that meets this interface.

.. function:: dumps(obj[, protocol])

   Return the pickled representation of the object as a :class:`bytes`
   object, instead of writing it to a file.

   The optional *protocol* argument tells the pickler to use the given protocol;
   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
   backward-incompatible protocol designed for Python 3.0.

   Specifying a negative protocol version selects the highest protocol version
   supported.  The higher the protocol used, the more recent the version of
   Python needed to read the pickle produced.

.. function:: load(file, [\*, encoding="ASCII", errors="strict"])

   Read a pickled object representation from the open file object *file* and
   return the reconstituted object hierarchy specified therein.  This is
   equivalent to ``Unpickler(file).load()``.

   The protocol version of the pickle is detected automatically, so no protocol
   argument is needed.  Bytes past the pickled object's representation are
   ignored.

   The argument *file* must have two methods, a read() method that takes an
   integer argument, and a readline() method that requires no arguments.  Both
   methods should return bytes.  Thus *file* can be a binary file object opened
   for reading, a BytesIO object, or any other custom object that meets this
   interface.

   Optional keyword arguments are encoding and errors, which are used to decode
   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
   'strict', respectively.

.. function:: loads(bytes_object, [\*, encoding="ASCII", errors="strict"])

   Read a pickled object hierarchy from a :class:`bytes` object and return the
   reconstituted object hierarchy specified therein

   The protocol version of the pickle is detected automatically, so no protocol
   argument is needed.  Bytes past the pickled object's representation are
   ignored.

   Optional keyword arguments are encoding and errors, which are used to decode
   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
   'strict', respectively.


The :mod:`pickle` module defines three exceptions:

.. exception:: PickleError

   Common base class for the other pickling exceptions. It inherits
   :exc:`Exception`.

.. exception:: PicklingError

   Error raised when an unpicklable object is encountered by :class:`Pickler`.
   It inherits :exc:`PickleError`.

.. exception:: UnpicklingError

   Error raised when there a problem unpickling an object, such as a data
   corruption or a security violation. It inherits :exc:`PickleError`.

   Note that other exceptions may also be raised during unpickling, including
   (but not necessarily limited to) AttributeError, EOFError, ImportError, and
   IndexError.


The :mod:`pickle` module exports two classes, :class:`Pickler` and
:class:`Unpickler`:

.. class:: Pickler(file[, protocol])

   This takes a binary file for writing a pickle data stream.

   The optional *protocol* argument tells the pickler to use the given protocol;
   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
   backward-incompatible protocol designed for Python 3.0.

   Specifying a negative protocol version selects the highest protocol version
   supported.  The higher the protocol used, the more recent the version of
   Python needed to read the pickle produced.

   The *file* argument must have a write() method that accepts a single bytes
   argument. It can thus be a file object opened for binary writing, a
   io.BytesIO instance, or any other custom object that meets this interface.

   .. method:: dump(obj)

      Write a pickled representation of *obj* to the open file object given in
      the constructor.

   .. method:: persistent_id(obj)

      Do nothing by default.  This exists so a subclass can override it.

      If :meth:`persistent_id` returns ``None``, *obj* is pickled as usual.  Any
      other value causes :class:`Pickler` to emit the returned value as a
      persistent ID for *obj*.  The meaning of this persistent ID should be
      defined by :meth:`Unpickler.persistent_load`.  Note that the value
      returned by :meth:`persistent_id` cannot itself have a persistent ID.

      See :ref:`pickle-persistent` for details and examples of uses.

   .. method:: clear_memo()

      Deprecated.  Use the :meth:`clear` method on the :attr:`memo`.  Clear the
      pickler's memo, useful when reusing picklers.

   .. attribute:: fast

      Enable fast mode if set to a true value.  The fast mode disables the usage
      of memo, therefore speeding the pickling process by not generating
      superfluous PUT opcodes.  It should not be used with self-referential
      objects, doing otherwise will cause :class:`Pickler` to recurse
      infinitely.

      Use :func:`pickletools.optimize` if you need more compact pickles.

   .. attribute:: memo

      Dictionary holding previously pickled objects to allow shared or
      recursive objects to pickled by reference as opposed to by value.


It is possible to make multiple calls to the :meth:`dump` method of the same
:class:`Pickler` instance.  These must then be matched to the same number of
calls to the :meth:`load` method of the corresponding :class:`Unpickler`
instance.  If the same object is pickled by multiple :meth:`dump` calls, the
:meth:`load` will all yield references to the same object.

Please note, this is intended for pickling multiple objects without intervening
modifications to the objects or their parts.  If you modify an object and then
pickle it again using the same :class:`Pickler` instance, the object is not
pickled again --- a reference to it is pickled and the :class:`Unpickler` will
return the old value, not the modified one.


.. class:: Unpickler(file, [\*, encoding="ASCII", errors="strict"])

   This takes a binary file for reading a pickle data stream.

   The protocol version of the pickle is detected automatically, so no
   protocol argument is needed.

   The argument *file* must have two methods, a read() method that takes an
   integer argument, and a readline() method that requires no arguments.  Both
   methods should return bytes.  Thus *file* can be a binary file object opened
   for reading, a BytesIO object, or any other custom object that meets this
   interface.

   Optional keyword arguments are encoding and errors, which are used to decode
   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
   'strict', respectively.

   .. method:: load()

      Read a pickled object representation from the open file object given in
      the constructor, and return the reconstituted object hierarchy specified
      therein. Bytes past the pickled object's representation are ignored.

   .. method:: persistent_load(pid)

      Raise an :exc:`UnpickingError` by default.

      If defined, :meth:`persistent_load` should return the object specified by
      the persistent ID *pid*. On errors, such as if an invalid persistent ID is
      encountered, an :exc:`UnpickingError` should be raised.

      See :ref:`pickle-persistent` for details and examples of uses.

   .. method:: find_class(module, name)

      Import *module* if necessary and return the object called *name* from it.
      Subclasses may override this to gain control over what type of objects can
      be loaded, potentially reducing security risks.


What can be pickled and unpickled?
----------------------------------

The following types can be pickled:

* ``None``, ``True``, and ``False``

* integers, floating point numbers, complex numbers

* strings, bytes, bytearrays

* tuples, lists, sets, and dictionaries containing only picklable objects

* functions defined at the top level of a module

* built-in functions defined at the top level of a module

* classes that are defined at the top level of a module

* instances of such classes whose :attr:`__dict__` or :meth:`__setstate__` is
  picklable  (see section :ref:`pickle-protocol` for details)

Attempts to pickle unpicklable objects will raise the :exc:`PicklingError`
exception; when this happens, an unspecified number of bytes may have already
been written to the underlying file. Trying to pickle a highly recursive data
structure may exceed the maximum recursion depth, a :exc:`RuntimeError` will be
raised in this case. You can carefully raise this limit with
:func:`sys.setrecursionlimit`.

Note that functions (built-in and user-defined) are pickled by "fully qualified"
name reference, not by value.  This means that only the function name is
pickled, along with the name of module the function is defined in.  Neither the
function's code, nor any of its function attributes are pickled.  Thus the
defining module must be importable in the unpickling environment, and the module
must contain the named object, otherwise an exception will be raised. [#]_

Similarly, classes are pickled by named reference, so the same restrictions in
the unpickling environment apply.  Note that none of the class's code or data is
pickled, so in the following example the class attribute ``attr`` is not
restored in the unpickling environment::

   class Foo:
       attr = 'a class attr'

   picklestring = pickle.dumps(Foo)

These restrictions are why picklable functions and classes must be defined in
the top level of a module.

Similarly, when class instances are pickled, their class's code and data are not
pickled along with them.  Only the instance data are pickled.  This is done on
purpose, so you can fix bugs in a class or add methods to the class and still
load objects that were created with an earlier version of the class.  If you
plan to have long-lived objects that will see many versions of a class, it may
be worthwhile to put a version number in the objects so that suitable
conversions can be made by the class's :meth:`__setstate__` method.


.. _pickle-protocol:

The pickle protocol
-------------------

This section describes the "pickling protocol" that defines the interface
between the pickler/unpickler and the objects that are being serialized.  This
protocol provides a standard way for you to define, customize, and control how
your objects are serialized and de-serialized.  The description in this section
doesn't cover specific customizations that you can employ to make the unpickling
environment slightly safer from untrusted pickle data streams; see section
:ref:`pickle-sub` for more details.


.. _pickle-inst:

Pickling and unpickling normal class instances
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
   single: __getinitargs__() (copy protocol)
   single: __init__() (instance constructor)

.. XXX is __getinitargs__ only used with old-style classes?
.. XXX update w.r.t Py3k's classes

When a pickled class instance is unpickled, its :meth:`__init__` method is
normally *not* invoked.  If it is desirable that the :meth:`__init__` method be
called on unpickling, an old-style class can define a method
:meth:`__getinitargs__`, which should return a *tuple* containing the arguments
to be passed to the class constructor (:meth:`__init__` for example).  The
:meth:`__getinitargs__` method is called at pickle time; the tuple it returns is
incorporated in the pickle for the instance.

.. index:: single: __getnewargs__() (copy protocol)

New-style types can provide a :meth:`__getnewargs__` method that is used for
protocol 2.  Implementing this method is needed if the type establishes some
internal invariants when the instance is created, or if the memory allocation is
affected by the values passed to the :meth:`__new__` method for the type (as it
is for tuples and strings).  Instances of a :term:`new-style class` :class:`C`
are created using ::

   obj = C.__new__(C, *args)


where *args* is the result of calling :meth:`__getnewargs__` on the original
object; if there is no :meth:`__getnewargs__`, an empty tuple is assumed.

.. index::
   single: __getstate__() (copy protocol)
   single: __setstate__() (copy protocol)
   single: __dict__ (instance attribute)

Classes can further influence how their instances are pickled; if the class
defines the method :meth:`__getstate__`, it is called and the return state is
pickled as the contents for the instance, instead of the contents of the
instance's dictionary.  If there is no :meth:`__getstate__` method, the
instance's :attr:`__dict__` is pickled.

Upon unpickling, if the class also defines the method :meth:`__setstate__`, it
is called with the unpickled state. [#]_  If there is no :meth:`__setstate__`
method, the pickled state must be a dictionary and its items are assigned to the
new instance's dictionary.  If a class defines both :meth:`__getstate__` and
:meth:`__setstate__`, the state object needn't be a dictionary and these methods
can do what they want. [#]_

.. warning::

   If :meth:`__getstate__` returns a false value, the :meth:`__setstate__`
   method will not be called.


Pickling and unpickling extension types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
   single: __reduce__() (pickle protocol)
   single: __reduce_ex__() (pickle protocol)
   single: __safe_for_unpickling__ (pickle protocol)

When the :class:`Pickler` encounters an object of a type it knows nothing about
--- such as an extension type --- it looks in two places for a hint of how to
pickle it.  One alternative is for the object to implement a :meth:`__reduce__`
method.  If provided, at pickling time :meth:`__reduce__` will be called with no
arguments, and it must return either a string or a tuple.

If a string is returned, it names a global variable whose contents are pickled
as normal.  The string returned by :meth:`__reduce__` should be the object's
local name relative to its module; the pickle module searches the module
namespace to determine the object's module.

When a tuple is returned, it must be between two and five elements long.
Optional elements can either be omitted, or ``None`` can be provided as their
value.  The contents of this tuple are pickled as normal and used to
reconstruct the object at unpickling time.  The semantics of each element are:

* A callable object that will be called to create the initial version of the
  object.  The next element of the tuple will provide arguments for this callable,
  and later elements provide additional state information that will subsequently
  be used to fully reconstruct the pickled data.

  In the unpickling environment this object must be either a class, a callable
  registered as a "safe constructor" (see below), or it must have an attribute
  :attr:`__safe_for_unpickling__` with a true value. Otherwise, an
  :exc:`UnpicklingError` will be raised in the unpickling environment.  Note that
  as usual, the callable itself is pickled by name.

* A tuple of arguments for the callable object, not ``None``.

* Optionally, the object's state, which will be passed to the object's
  :meth:`__setstate__` method as described in section :ref:`pickle-inst`.  If the
  object has no :meth:`__setstate__` method, then, as above, the value must be a
  dictionary and it will be added to the object's :attr:`__dict__`.

* Optionally, an iterator (and not a sequence) yielding successive list items.
  These list items will be pickled, and appended to the object using either
  ``obj.append(item)`` or ``obj.extend(list_of_items)``.  This is primarily used
  for list subclasses, but may be used by other classes as long as they have
  :meth:`append` and :meth:`extend` methods with the appropriate signature.
  (Whether :meth:`append` or :meth:`extend` is used depends on which pickle
  protocol version is used as well as the number of items to append, so both must
  be supported.)

* Optionally, an iterator (not a sequence) yielding successive dictionary items,
  which should be tuples of the form ``(key, value)``.  These items will be
  pickled and stored to the object using ``obj[key] = value``. This is primarily
  used for dictionary subclasses, but may be used by other classes as long as they
  implement :meth:`__setitem__`.

It is sometimes useful to know the protocol version when implementing
:meth:`__reduce__`.  This can be done by implementing a method named
:meth:`__reduce_ex__` instead of :meth:`__reduce__`. :meth:`__reduce_ex__`, when
it exists, is called in preference over :meth:`__reduce__` (you may still
provide :meth:`__reduce__` for backwards compatibility).  The
:meth:`__reduce_ex__` method will be called with a single integer argument, the
protocol version.

The :class:`object` class implements both :meth:`__reduce__` and
:meth:`__reduce_ex__`; however, if a subclass overrides :meth:`__reduce__` but
not :meth:`__reduce_ex__`, the :meth:`__reduce_ex__` implementation detects this
and calls :meth:`__reduce__`.

An alternative to implementing a :meth:`__reduce__` method on the object to be
pickled, is to register the callable with the :mod:`copyreg` module.  This
module provides a way for programs to register "reduction functions" and
constructors for user-defined types.   Reduction functions have the same
semantics and interface as the :meth:`__reduce__` method described above, except
that they are called with a single argument, the object to be pickled.

The registered constructor is deemed a "safe constructor" for purposes of
unpickling as described above.


.. _pickle-persistent:

Pickling and unpickling external objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
   single: persistent_id (pickle protocol)
   single: persistent_load (pickle protocol)

For the benefit of object persistence, the :mod:`pickle` module supports the
notion of a reference to an object outside the pickled data stream.  Such
objects are referenced by a "persistent id", which is just an arbitrary string
of printable ASCII characters. The resolution of such names is not defined by
the :mod:`pickle` module; it will delegate this resolution to user defined
functions on the pickler and unpickler.

To define external persistent id resolution, you need to set the
:attr:`persistent_id` attribute of the pickler object and the
:attr:`persistent_load` attribute of the unpickler object.

To pickle objects that have an external persistent id, the pickler must have a
custom :func:`persistent_id` method that takes an object as an argument and
returns either ``None`` or the persistent id for that object.  When ``None`` is
returned, the pickler simply pickles the object as normal.  When a persistent id
string is returned, the pickler will pickle that string, along with a marker so
that the unpickler will recognize the string as a persistent id.

To unpickle external objects, the unpickler must have a custom
:func:`persistent_load` function that takes a persistent id string and returns
the referenced object.

Here's a silly example that *might* shed more light::

   import pickle
   from io import StringIO

   src = StringIO()
   p = pickle.Pickler(src)

   def persistent_id(obj):
       if hasattr(obj, 'x'):
           return 'the value %d' % obj.x
       else:
           return None

   p.persistent_id = persistent_id

   class Integer:
       def __init__(self, x):
           self.x = x
       def __str__(self):
           return 'My name is integer %d' % self.x

   i = Integer(7)
   print(i)
   p.dump(i)

   datastream = src.getvalue()
   print(repr(datastream))
   dst = StringIO(datastream)

   up = pickle.Unpickler(dst)

   class FancyInteger(Integer):
       def __str__(self):
           return 'I am the integer %d' % self.x

   def persistent_load(persid):
       if persid.startswith('the value '):
           value = int(persid.split()[2])
           return FancyInteger(value)
       else:
           raise pickle.UnpicklingError('Invalid persistent id')

   up.persistent_load = persistent_load

   j = up.load()
   print(j)


.. BAW: pickle supports something called inst_persistent_id()
   which appears to give unknown types a second shot at producing a persistent
   id.  Since Jim Fulton can't remember why it was added or what it's for, I'm
   leaving it undocumented.


.. _pickle-sub:

Subclassing Unpicklers
----------------------

.. index::
   single: load_global() (pickle protocol)
   single: find_global() (pickle protocol)

By default, unpickling will import any class that it finds in the pickle data.
You can control exactly what gets unpickled and what gets called by customizing
your unpickler.

You need to derive a subclass from :class:`Unpickler`, overriding the
:meth:`load_global` method.  :meth:`load_global` should read two lines from the
pickle data stream where the first line will the name of the module containing
the class and the second line will be the name of the instance's class.  It then
looks up the class, possibly importing the module and digging out the attribute,
then it appends what it finds to the unpickler's stack.  Later on, this class
will be assigned to the :attr:`__class__` attribute of an empty class, as a way
of magically creating an instance without calling its class's
:meth:`__init__`. Your job (should you choose to accept it), would be to have
:meth:`load_global` push onto the unpickler's stack, a known safe version of any
class you deem safe to unpickle.  It is up to you to produce such a class.  Or
you could raise an error if you want to disallow all unpickling of instances.
If this sounds like a hack, you're right.  Refer to the source code to make this
work.

The moral of the story is that you should be really careful about the source of
the strings your application unpickles.


.. _pickle-example:

Example
-------

For the simplest code, use the :func:`dump` and :func:`load` functions.  Note
that a self-referencing list is pickled and restored correctly. ::

   import pickle

   data1 = {'a': [1, 2.0, 3, 4+6j],
            'b': ("string", "string using Unicode features \u0394"),
            'c': None}

   selfref_list = [1, 2, 3]
   selfref_list.append(selfref_list)

   output = open('data.pkl', 'wb')

   # Pickle dictionary using protocol 2.
   pickle.dump(data1, output, 2)

   # Pickle the list using the highest protocol available.
   pickle.dump(selfref_list, output, -1)

   output.close()

The following example reads the resulting pickled data.  When reading a
pickle-containing file, you should open the file in binary mode because you
can't be sure if the ASCII or binary format was used. ::

   import pprint, pickle

   pkl_file = open('data.pkl', 'rb')

   data1 = pickle.load(pkl_file)
   pprint.pprint(data1)

   data2 = pickle.load(pkl_file)
   pprint.pprint(data2)

   pkl_file.close()

Here's a larger example that shows how to modify pickling behavior for a class.
The :class:`TextReader` class opens a text file, and returns the line number and
line contents each time its :meth:`readline` method is called. If a
:class:`TextReader` instance is pickled, all attributes *except* the file object
member are saved. When the instance is unpickled, the file is reopened, and
reading resumes from the last location. The :meth:`__setstate__` and
:meth:`__getstate__` methods are used to implement this behavior. ::

   #!/usr/local/bin/python

   class TextReader:
       """Print and number lines in a text file."""
       def __init__(self, file):
           self.file = file
           self.fh = open(file)
           self.lineno = 0

       def readline(self):
           self.lineno = self.lineno + 1
           line = self.fh.readline()
           if not line:
               return None
           if line.endswith("\n"):
               line = line[:-1]
           return "%d: %s" % (self.lineno, line)

       def __getstate__(self):
           odict = self.__dict__.copy() # copy the dict since we change it
           del odict['fh']              # remove filehandle entry
           return odict

       def __setstate__(self, dict):
           fh = open(dict['file'])      # reopen file
           count = dict['lineno']       # read from file...
           while count:                 # until line count is restored
               fh.readline()
               count = count - 1
           self.__dict__.update(dict)   # update attributes
           self.fh = fh                 # save the file object

A sample usage might be something like this::

   >>> import TextReader
   >>> obj = TextReader.TextReader("TextReader.py")
   >>> obj.readline()
   '1: #!/usr/local/bin/python'
   >>> obj.readline()
   '2: '
   >>> obj.readline()
   '3: class TextReader:'
   >>> import pickle
   >>> pickle.dump(obj, open('save.p', 'wb'))

If you want to see that :mod:`pickle` works across Python processes, start
another Python session, before continuing.  What follows can happen from either
the same process or a new process. ::

   >>> import pickle
   >>> reader = pickle.load(open('save.p', 'rb'))
   >>> reader.readline()
   '4:     """Print and number lines in a text file."""'


.. seealso::

   Module :mod:`copyreg`
      Pickle interface constructor registration for extension types.

   Module :mod:`shelve`
      Indexed databases of objects; uses :mod:`pickle`.

   Module :mod:`copy`
      Shallow and deep object copying.

   Module :mod:`marshal`
      High-performance serialization of built-in types.


.. rubric:: Footnotes

.. [#] Don't confuse this with the :mod:`marshal` module

.. [#] The exception raised will likely be an :exc:`ImportError` or an
   :exc:`AttributeError` but it could be something else.

.. [#] These methods can also be used to implement copying class instances.

.. [#] This protocol is also used by the shallow and deep copying operations defined in
   the :mod:`copy` module.