Abstract

Python source code is compiled down to something called bytecode. This bytecode must implement enough semantics to perform the actions required by the Language Reference [1]. As such, knowing how to add, remove, or change the bytecode is important to do properly when changing the abilities of the Python language. This PEP covers how to accomplish this in the CPython implementation of the language (referred to as simply "Python" for the rest of this PEP).

Rationale

While changing Python's bytecode is not a frequent occurence, it still happens. Having the required steps documented in a single location should make experimentation with the bytecode easier since it is not necessarily obvious what the steps are to change the bytecode.

This PEP, paired with PEP 306 [2], should provide enough basic guidelines for handling any changes performed to the Python language itself in terms of syntactic changes that introduce new semantics.

Checklist

This is a rough checklist of what files need to change and how they are involved with the bytecode. All paths are given from the viewpoint of /cvsroot/python/dist/src from CVS). This list should not be considered exhaustive nor to cover all possible situations.

• Include/opcode.h

  This include file lists all known opcodes and associates each opcode name with a unique number. When adding a new opcode it is important to take note of the HAVE_ARGUMENT value. This #define's value specifies the value at which all opcodes greater than HAVE_ARGUMENT are expected to take an argument to the opcode.

• Lib/opcode.py

  Lists all of the opcodes and their associated value. Used by the dis module [3] to map bytecode values to their names.

• Python/ceval.c

  Contains the main interpreter loop. Code to handle the evalution of an opcode goes here.

• Python/compile.c

  To make sure an opcode is actually used, this file must be altered. The emitting of all bytecode occurs here.

• Lib/compiler/pyassem.py, Lib/compiler/pycodegen.py

  The 'compiler' package [4] needs to be altered to also reflect any changes to the bytecode.

• Doc/lib/libdis.tex

  The documentation [6] for the dis module contains a complete list of all the opcodes.

• Python/import.c

  Defines the magic word (named MAGIC) used in .pyc files to detect if the bytecode used matches the one used by the version of Python running. This number needs to be changed to make sure that the running interpreter does not try to execute bytecode that it does not know about.

Suggestions for bytecode development

A few things can be done to make sure that development goes smoothly when experimenting with Python's bytecode. One is to delete all .py(c|o) files after each semantic change to Python/compile.c . That way all files will use any bytecode changes.

Make sure to run the entire testing suite [5]. Since the regrtest.py driver recompiles all source code before a test is run it acts a good test to make sure that no existing semantics are broken.

Running parrotbench [7] is also a good way to make sure existing semantics are not broken; this benchmark is practically a compliance test.

Previous experiments

This section lists known bytecode experiments that have not gone into Python.

Skip Montanaro presented a paper at a Python workshop on a peephole optimizer [8].

Michael Hudson has a non-active SourceForge project named Bytecodehacks [9] that provides functionality for playing with bytecode directly.

An opcode to combine the functionality of LOAD_ATTR/CALL_FUNCTION was created named CALL_ATTR [10]. Currently only works for classic classes and for new-style classes rough benchmarking showed an actual slowdown thanks to having to support both classic and new-style classes.

References

Copyright

This document has been placed in the public domain.