Assembling Bytecode - PyCode New

Question

The following code produces bytecode output as string. I want to replace some lines of the bytecode and reassemble and exec it. Do I need something like the ByteCodeAssember or can I do this with PyCode New?

http://docs.python.org/2/c-api/code.html#PyCode_New

http://pypi.python.org/pypi/BytecodeAssembler

output

********** code ********** 
type: <type 'str'> 
def test(a,b): 
 return a*b 
test(2,3) 
********** compiles into the bytecode ********** 
type: <type 'code'> 
 1 0 LOAD_CONST 0 (<code object test at 0x101d1ca30, file "<string>", line 1>) 
 3 MAKE_FUNCTION 0 
 6 STORE_NAME 0 (test) 
 3 9 LOAD_NAME 0 (test) 
 12 LOAD_CONST 1 (2) 
 15 LOAD_CONST 2 (3) 
 18 CALL_FUNCTION 2 
 21 POP_TOP 
 22 LOAD_CONST 3 (None) 
 25 RETURN_VALUE 3 
********** bytecode **********
'd\x00\x00\x84\x00\x00Z\x00\x00e\x00\x00d\x01\x00d\x02\x00\x83\x02\x00\x01d\x03\x00S'

code

import dis 
import sys 
import inspect 
import new 
class writer: 
 def __init__(self): 
 self.s = "" 
 def write(self,text): 
 #print ': ',text 
 self.s += text 
#save stdout 
origstdout = sys.stdout 
w = writer() 
sys.stdout = w 
s = "def test(a,b):\n\treturn a*b\ntest(2,3)" 
c = compile(s,'<string>','exec') 
# dis calls stdout, so output is in w 
bytecode = dis.dis(c) 
sys.stdout = origstdout 
def h(x): 
 print '*'*10 + '\t' + x + '\t' + '*'*10 + '\n'*1 
h('code') 
print 'type: '+str(type(s)) 
print s + '\n' 
h('compiles into the bytecode') 
print 'type: '+str(type(c)) 
print w.s
h('bytecode') 
print repr(c.co_code) 

• martineau: I've seen other answers, like this one where people have modified bytecode without using PyCode at all which might help you. Note esp the add_self() function.

Answer 1

Byteplay is a nice wrapper for python code objects. It has its own code class. Code objects is CPython specific and more complicated. To get started certainly better to play with byteplay first.

from byteplay import * 
def foo(): 
 x = 10 
 print 'inside ',x 
 return 42 
c = Code.from_code(foo.func_code) 
print c.code 
ret = foo() 
print 'outside: ',ret 
c.code[10] = (LOAD_CONST,1000) 
foo.func_code = c.to_code() 
print '*'*20 
print c.code 
ret = foo() 
print 'outside: ',ret 

produces

 4 1 LOAD_CONST 10
 2 STORE_FAST x
 5 4 LOAD_CONST 'inside '
 5 PRINT_ITEM 
 6 LOAD_FAST x
 7 PRINT_ITEM 
 8 PRINT_NEWLINE 
 6 10 LOAD_CONST 42
 11 RETURN_VALUE 
inside 10
outside: 42
********************
 4 1 LOAD_CONST 10
 2 STORE_FAST x
 5 4 LOAD_CONST 'inside '
 5 PRINT_ITEM 
 6 LOAD_FAST x
 7 PRINT_ITEM 
 8 PRINT_NEWLINE 
 6 10 LOAD_CONST 1000
 11 RETURN_VALUE 
inside 10
outside: 1000