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Python cross-version byte-code assembler

Project description

Pypi Installs Latest Version Supported Python Versions


NOTE: this is in beta

A Cross-Python bytecode Assembler


The Python xasm module has routines for assembly, and has a command to assemble bytecode for several different versions of Python.

Here are some potential uses:

  • Make small patches to existing Python bytecode when you don’t have source
  • Craft custom and efficient bytecode
  • Write an instruction-level optimizing compiler
  • Experiment with and learn about Python bytecode
  • Foil uncompyle6 so that it can’t disassemble bytecode (at least for now)

This will support bytecodes from Python version 1.0 to 3.8 or so.

The code requires Python 2.7 or later.

Assembly files

More detail will be filled in, but some principles:

  • Preferred extension for Python assembly is .pyasm
  • assembly is designed to work with the output of pydisasm –asm
  • Assembly file labels are at the beginning of the line and end in a colon, e.g. END_IF
  • instruction offsets in the assembly file are ignored and don’t need to be entered
  • in those instructions that refer to offsets, if the if the operand is an int, exactly that value will be used for the operand. Otherwise we will look for labels and match up with that


The standard Python routine:

pip install -e .
pip install -r requirements-dev.txt

A GNU makefile is also provided so make install (possibly as root or sudo) will do the steps above.


make check

A GNU makefile has been added to smooth over setting running the right command, and running tests from fastest to slowest.

If you have remake installed, you can see the list of all tasks including tests via remake --tasks.

Example Assembly File

For this Python source code:

def five():
    return 5


Here is an assembly for the above:

# Python bytecode 3.6 (3379)

# Method Name:       five
# Filename:          /tmp/
# Argument count:    0
# Kw-only arguments: 0
# Number of locals:  0
# Stack size:        1
# Flags:             0x00000043 (NOFREE | NEWLOCALS | OPTIMIZED)
# First Line:        1
# Constants:
#    0: None
#    1: 5
            LOAD_CONST           (5)

# Method Name:       <module>
# Filename:          /tmp/
# Argument count:    0
# Kw-only arguments: 0
# Number of locals:  0
# Stack size:        2
# Flags:             0x00000040 (NOFREE)
# First Line:        1
# Constants:
#    0: <code object five at 0x0000>
#    1: 'five'
#    2: None
# Names:
#    0: five
#    1: print
            LOAD_CONST           0 (<code object five at 0x0000>)
            LOAD_CONST           ('five')
            MAKE_FUNCTION        0
            STORE_NAME           (five)

            LOAD_NAME            (print)
            LOAD_NAME            (five)
            CALL_FUNCTION        0
            CALL_FUNCTION        1
            LOAD_CONST           (None)

The above can be created automatically from Python source code using the pydisasm command from xdis:

pydisasm --format xasm /tmp/five.pyc

In the example above though, I have shortend and simplified the result.


To create a python bytecode file from an assemble file, run:


For usage help, type pyc-xasm –help.

To convert a python bytecode from one bytecode to another, run:


For usage help, type pyc-convert –help.

See Also

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