[Python-Dev] ANN: byteplay - a bytecode assembler/disassembler (original) (raw)

Noam Raphael noamraph at gmail.com
Mon Aug 14 21🔞45 CEST 2006

Hello,

I wanted to tell you that I wrote a Python bytecode assembler/disassembler, and would be happy if people tried it and said what they think.

I send this message to this list because this module deals with pretty low-level Python, so I thought it might interest the audience here. If I was wrong - please forgive me.

The idea is to define an object which is equivalent to Python's code object, but which is easy to work with. To explain what I mean, I'll show a quick example. We can define this stupid function:

def f(a, b): ... print (a, b)

We can convert it to an equivalent object, and see how it stores the byte code:

from byteplay import * c = Code.fromcode(f.funccode) from pprint import pprint; pprint(c.code) [(SetLineno, 2), (LOAD_FAST, 'a'), (LOAD_FAST, 'b'), (BUILD_TUPLE, 2), (PRINT_ITEM, None), (PRINT_NEWLINE, None), (LOAD_CONST, None), (RETURN_VALUE, None)]

We can change the bytecode easily, and see what happens:

c.code[3:3] = [(ROTTWO, None)] f.funccode = c.tocode() f(3, 5) (5, 3)

The idea is basically the same as that of Michael Hudson's bytecodehacks, but this one works with Python 2.4 and 2.5. I also think that it's simpler to use. I borrowed some code from Phillip J. Eby's peak.util.assembler - the main difference between his package and mine is that mine lets you play with existing bytecode, not only create new code objects.

I learned a lot about Python's bytecode from writing this, and I think that other may learn from it as well - I think it's much easier to understand how code objects work by understanding equivalent objects which were meant to be as simple as possible, instead of as fast as possible.

I think it got pretty good testing - I patched import so that after a module is imported, all function objects (found by the gc module) were disassembled and assembled again. I then managed to get the complete test suite to pass!

You can download the module from http://byteplay.googlecode.com/svn/trunk/byteplay.py . I wrote a documentation (which goes into some length in purpose of explaining how bytecode works). It's on the wiki: http://wiki.python.org/moin/ByteplayDoc .

I even thought that it might get to the standard library, because it seemed to me to be a pretty good documentation of bytecode details, and because it has a pretty straightforward interface. It also makes meddling with bytecode much less dangerous - for example, you can see Raymond's original recipe for binding constants at compile time (I reposted it at http://python.pastebin.com/768312) and how using byteplay makes it simple (posted at http://python.pastebin.com/768318 so you can view the diff). But, of course - it's up to you. I will be entirely satisfied if people simply find it useful.

Have a good day, Noam

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