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python-peps / pep-3154.txt

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PEP: 3154
Title: Pickle protocol version 4
Version: $Revision$
Last-Modified: $Date$
Author: Antoine Pitrou <solipsis@pitrou.net>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2011-08-11
Python-Version: 3.3
Post-History:
Resolution: TBD


Abstract
========

Data serialized using the pickle module must be portable across Python
versions.  It should also support the latest language features as well
as implementation-specific features.  For this reason, the pickle
module knows about several protocols (currently numbered from 0 to 3),
each of which appeared in a different Python version.  Using a
low-numbered protocol version allows to exchange data with old Python
versions, while using a high-numbered protocol allows access to newer
features and sometimes more efficient resource use (both CPU time
required for (de)serializing, and disk size / network bandwidth
required for data transfer).


Rationale
=========

The latest current protocol, coincidentally named protocol 3, appeared
with Python 3.0 and supports the new incompatible features in the
language (mainly, unicode strings by default and the new bytes
object).  The opportunity was not taken at the time to improve the
protocol in other ways.

This PEP is an attempt to foster a number of small incremental
improvements in a future new protocol version.  The PEP process is
used in order to gather as many improvements as possible, because the
introduction of a new protocol version should be a rare occurrence.


Improvements in discussion
==========================

64-bit compatibility for large objects
--------------------------------------

Current protocol versions export object sizes for various built-in
types (str, bytes) as 32-bit ints.  This forbids serialization of
large data [1]_.  New opcodes are required to support very large bytes
and str objects.

Native opcodes for sets and frozensets
--------------------------------------

Many common built-in types (such as str, bytes, dict, list, tuple)
have dedicated opcodes to improve resource consumption when
serializing and deserializing them; however, sets and frozensets
don't.  Adding such opcodes would be an obvious improvement.  Also,
dedicated set support could help remove the current impossibility of
pickling self-referential sets [2]_.

Calling __new__ with keyword arguments
--------------------------------------

Currently, classes whose __new__ mandates the use of keyword-only
arguments can not be pickled (or, rather, unpickled) [3]_.  Both a new
special method (``__getnewargs_ex__`` ?) and a new opcode (NEWOBJEX ?)
are needed.

Serializing more "lookupable" objects
-------------------------------------

For some kinds of objects, it only makes sense to serialize them by name
(for example classes and functions).  By default, pickle is only able to
serialize module-global functions and classes by name.  Supporting other
kinds of objects, such as unbound methods [4]_, is a common request.
Actually, third-party support for some of them, such as bound methods,
is implemented in the multiprocessing module [5]_.

:pep:`3155` now makes it possible to lookup many more objects by name.
Generalizing the GLOBAL opcode to accept dot-separated names, or adding
a special GETATTR opcode, would allow the standard pickle implementation
to support, in an efficient way, all those kinds of objects.

Binary encoding for all opcodes
-------------------------------

The GLOBAL opcode, which is still used in protocol 3, uses the
so-called "text" mode of the pickle protocol, which involves looking
for newlines in the pickle stream.  Looking for newlines is difficult
to optimize on a non-seekable stream, and therefore a new version of
GLOBAL (BINGLOBAL?)  could use a binary encoding instead.

It seems that all other opcodes emitted when using protocol 3 already
use binary encoding.

Better string encoding
----------------------

Short str objects currently have their length coded as a 4-bytes
integer, which is wasteful.  A specific opcode with a 1-byte length
would make many pickles smaller.


Acknowledgments
===============

(...)


References
==========

.. [1] "pickle not 64-bit ready":
   http://bugs.python.org/issue11564

.. [2] "Cannot pickle self-referencing sets":
   http://bugs.python.org/issue9269

.. [3] "pickle/copyreg doesn't support keyword only arguments in __new__":
   http://bugs.python.org/issue4727

.. [4] "pickle should support methods":
   http://bugs.python.org/issue9276

.. [5] Lib/multiprocessing/forking.py:
   http://hg.python.org/cpython/file/baea9f5f973c/Lib/multiprocessing/forking.py#l54


Copyright
=========

This document has been placed in the public domain.



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