Added PEP 339, "How to Change CPython's Bytecode", by Brett Cannon.
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Brett Cannon
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@ -58,6 +58,7 @@ Index by Category
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I 291 Backward Compatibility for Standard Library Norwitz
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I 306 How to Change Python's Grammar Hudson
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I 333 Python Web Server Gateway Interface v1.0 Eby
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I 339 How to Change CPython's Bytecode Cannon
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I 3000 Python 3.0 Plans Kuchling, Cannon
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Accepted PEPs (accepted; may not be implemented yet)
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@ -372,6 +373,7 @@ Numerical Index
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S 336 Make None Callable McClelland
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S 337 Logging Usage in the Standard Library Dubner
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S 338 Executing modules inside packages with '-m' Coghlan
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I 339 How to Change CPython's Bytecode Cannon
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SR 666 Reject Foolish Indentation Creighton
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S 754 IEEE 754 Floating Point Special Values Warnes
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I 3000 Python 3.0 Plans Kuchling, Cannon
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@ -401,7 +403,7 @@ Owners
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Batista, Facundo facundo@taniquetil.com.ar
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Baxter, Anthony anthony@interlink.com.au
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Bellman, Thomas bellman+pep-divmod@lysator.liu.se
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Cannon, Brett drifty@alum.berkeley.edu
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Cannon, Brett brett@python.org
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Carlson, Josiah jcarlson@uci.edu
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Carroll, W Isaac icarroll@pobox.com
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Coghlan, Nick ncoghlan@email.com
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@ -0,0 +1,170 @@
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PEP: 339
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Title: How to Change CPython's Bytecode
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Version: $Revision$
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Last-Modified: $Date$
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Author: Brett Cannon <brett@python.org>
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Status: Active
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Type: Informational
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Content-Type: text/x-rst
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Created: 02-Feb-2005
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Post-History: 02-Feb-2005
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Abstract
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========
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Python source code is compiled down to something called bytecode. This
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bytecode must implement enough semantics to perform the actions required by the
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Language Reference [#lang_ref]_. As such, knowing how to add, remove, or change
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the bytecode is important to do properly when changing the abilities of the
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Python language.
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This PEP covers how to accomplish this in the CPython implementation of the
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language (referred to as simply "Python" for the rest of this PEP).
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.. warning::
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The guidelines outlined in this PEP apply to Python 2.4 and earlier.
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Current plans for Python 2.5 will lead to a significant change in how
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Python's bytecode is handled.
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This PEP will be updated once these planned changes are committed into
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CVS.
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Rationale
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=========
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While changing Python's bytecode is not a frequent occurence, it still happens.
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Having the required steps documented in a single location should make
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experimentation with the bytecode easier since it is not necessarily obvious
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what the steps are to change the bytecode.
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This PEP, paired with PEP 306 [#PEP-306]_, should provide enough basic
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guidelines for handling any changes performed to the Python language itself in
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terms of syntactic changes that introduce new semantics.
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Checklist
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=========
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This is a rough checklist of what files need to change and how they are
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involved with the bytecode. All paths are given from the viewpoint of
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``/cvsroot/python/dist/src`` from CVS). This list should not be considered
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exhaustive nor to cover all possible situations.
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- ``Include/opcode.h``
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This include file lists all known opcodes and associates each opcode
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name with
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a unique number. When adding a new opcode it is important to take note
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of the ``HAVE_ARGUMENT`` value. This ``#define``'s value specifies the
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value at which all opcodes greater than ``HAVE_ARGUMENT`` are expected
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to take an argument to the opcode.
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- ``Lib/opcode.py``
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Lists all of the opcodes and their associated value. Used by the dis
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module [#dis]_ to map bytecode values to their names.
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- ``Python/ceval.c``
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Contains the main interpreter loop. Code to handle the evalution of an
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opcode goes here.
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- ``Python/compile.c``
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To make sure an opcode is actually used, this file must be altered.
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The emitting of all bytecode occurs here.
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- ``Lib/compiler/pyassem.py``, ``Lib/compiler/pycodegen.py``
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The 'compiler' package [#compiler]_ needs to be altered to also reflect
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any changes to the bytecode.
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- ``Doc/lib/libdis.tex``
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The documentation [#opcode_list]_ for the dis module contains a complete
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list of all the opcodes.
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- ``Python/import.c``
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Defines the magic word (named ``MAGIC``) used in .pyc files to detect if
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the bytecode used matches the one used by the version of Python running.
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This number needs to be changed to make sure that the running
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interpreter does not try to execute bytecode that it does not know
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about.
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Suggestions for bytecode development
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====================================
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A few things can be done to make sure that development goes smoothly when
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experimenting with Python's bytecode. One is to delete all .py(c|o) files
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after each semantic change to Python/compile.c . That way all files will use
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any bytecode changes.
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Make sure to run the entire testing suite [#test-suite]_. Since the
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``regrtest.py`` driver recompiles all source code before a test is run it acts
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a good test to make sure that no existing semantics are broken.
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Running parrotbench [#parrotbench]_ is also a good way to make sure existing
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semantics are not broken; this benchmark is practically a compliance test.
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Previous experiments
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====================
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This section lists known bytecode experiments that have not gone into Python.
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Skip Montanaro presented a paper at a Python workshop on a peephole optimizer
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[#skip-peephole]_.
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Michael Hudson has a non-active SourceForge project named Bytecodehacks
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[#Bytecodehacks]_ that provides functionality for playing with bytecode
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directly.
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An opcode to combine the functionality of LOAD_ATTR/CALL_FUNCTION was created
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named CALL_ATTR [#CALL_ATTR]_. Currently only works for classic classes and
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for new-style classes rough benchmarking showed an actual slowdown thanks to
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having to support both classic and new-style classes.
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References
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==========
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.. [#lang_ref] Python Language Reference, van Rossum & Drake
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(http://docs.python.org/ref/ref.html)
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.. [#PEP-306] PEP 306, How to Change Python's Grammar, Hudson
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(http://www.python.org/peps/pep-0306.html)
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.. [#dis] dis Module
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(http://docs.python.org/lib/module-dis.html)
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.. [#compiler] 'compiler' Package
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(http://docs.python.org/lib/module-compiler.html)
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.. [#test-suite] 'test' Package
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(http://docs.python.org/lib/module-test.html)
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.. [#opcode_list] Python Byte Code Instructions
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(http://docs.python.org/lib/bytecodes.html)
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.. [#parrotbench] Parrotbench
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(ftp://ftp.python.org/pub/python/parrotbench/parrotbench.tgz,
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http://mail.python.org/pipermail/python-dev/2003-December/041527.html)
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.. [#skip-peephole] Skip Montanaro's Peephole Optimizer Paper
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(http://www.foretec.com/python/workshops/1998-11/proceedings/papers/montanaro/montanaro.html)
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.. [#Bytecodehacks] Bytecodehacks Project
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(http://bytecodehacks.sourceforge.net/bch-docs/bch/index.html)
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.. [#CALL_ATTR] CALL_ATTR opcode
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(http://www.python.org/sf/709744)
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Copyright
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=========
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This document has been placed in the public domain.
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..
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