DevTut

# The dis module

# Constants in the dis module

EXTENDED_ARG = 145 # All opcodes greater than this have 2 operands
HAVE_ARGUMENT = 90 # All opcodes greater than this have at least 1 operands

cmp_op = ('<', '<=', '==', '!=', '>', '>=', 'in', 'not in', 'is', 'is ...
       # A list of comparator id's. The indecies are used as operands in some opcodes

# All opcodes in these lists have the respective types as there operands
hascompare = [107]
hasconst = [100]
hasfree = [135, 136, 137]
hasjabs = [111, 112, 113, 114, 115, 119]
hasjrel = [93, 110, 120, 121, 122, 143]
haslocal = [124, 125, 126]
hasname = [90, 91, 95, 96, 97, 98, 101, 106, 108, 109, 116]

# A map of opcodes to ids
opmap = {'BINARY_ADD': 23, 'BINARY_AND': 64, 'BINARY_DIVIDE': 21, 'BIN...
# A map of ids to opcodes
opname = ['STOP_CODE', 'POP_TOP', 'ROT_TWO', 'ROT_THREE', 'DUP_TOP', '...

# What is Python bytecode?

Python is a hybrid interpreter. When running a program, it first assembles it into bytecode which can then be run in the Python interpreter (also called a Python virtual machine). The dis module in the standard library can be used to make the Python bytecode human-readable by disassembling classes, methods, functions, and code objects.

>>> def hello():
...     print "Hello, World"
...
>>> dis.dis(hello)
  2           0 LOAD_CONST               1 ('Hello, World')
              3 PRINT_ITEM
              4 PRINT_NEWLINE
              5 LOAD_CONST               0 (None)
              8 RETURN_VALUE

The Python interpreter is stack-based and uses a first-in last-out system.

Each operation code (opcode) in the Python assembly language (the bytecode) takes a fixed number of items from the stack and returns a fixed number of items to the stack. If there aren't enough items on the stack for an opcode, the Python interpreter will crash, possibly without an error message.

# Disassembling modules

To disassemble a Python module, first this has to be turned into a .pyc file (Python compiled). To do this, run

python -m compileall <file>.py

Then in an interpreter, run

import dis
import marshal
with open("<file>.pyc", "rb") as code_f:
    code_f.read(8) # Magic number and modification time
    code = marshal.load(code_f) # Returns a code object which can be disassembled
    dis.dis(code) # Output the disassembly

This will compile a Python module and output the bytecode instructions with dis. The module is never imported so it is safe to use with untrusted code.

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