Reverse engineering python 3

Question

I was wondering if there is a way to get around this error. Any help would be appreciated!

TypeError: can't concat bytes to str

import marshal, imp

f=open('PYTHONSCRIPT','rb')
f.seek(28)  # Skip the header, you have to know the header size beforehand

ob=marshal.load(f)

for i in range(0,len(ob)):
    open(str(i)+'.pyc','wb').write(imp.get_magic() + '\0'*4 + marshal.dumps(ob[i]))

f.close()

open(str(i)+'.pyc','wb').write(imp.get_magic() + '\0'*4 + marshal.dumps(ob[i]))

Possible duplicate of [Python 3 TypeError: must be str, not bytes with sys.stdout.write()](http://stackoverflow.com/questions/21689365/python-3-typeerror-must-be-str-not-bytes-with-sys-stdout-write) — Martin Tournoij, Mar 11 '16 at 06:34
The `str` and `bytes` types are not compatible. You must convert one of them. The above answer should give you the information and details you need. — Martin Tournoij, Mar 11 '16 at 06:35
@Carpetsmoker not even that, need to have something with `str` + `bytes` — Antti Haapala -- Слава Україні, Mar 11 '16 at 06:53

score 2 · Answer 1 · answered Mar 11 '16 at 06:49

2

'\0'*4 is a str, use b'\0' * 4 to get the needed bytes value.

answered Mar 11 '16 at 06:49

Antti Haapala -- Слава Україні

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score 1 · Accepted Answer · answered Mar 11 '16 at 06:50

1

Your problem is that you are trying to concatenate two bytes and a str. This is not possible in python3 because python3 makes a clear distinction between bytes and strings (a good thing given the somewhat fuzzy distinctions between str and unicode in python2). I think what you want is probably the following:

import marshal, imp

f=open('PYTHONSCRIPT','rb')
f.seek(28)  # Skip the header, you have to know the header size beforehand

ob=marshal.load(f)

for i in range(0,len(ob)):
    with open(str(i)+'.pyc','wb') as my_file:
        my_file.write(imp.get_magic() + b'\0'*4 + marshal.dumps(ob[i]))

f.close()

with open(str(i)+'.pyc','wb') as my_file:
    my_file.write(imp.get_magic() + b'\0'*4 + marshal.dumps(ob[i]))

The b before the strings is a marker that tells python that the strings are byte strings instead of str strings.

Note also that I added with ... as ...: which will make sure that your files get deterministically closed right away even in non-CPython implementations (PyPy, Jython, IronPython, etc).

answered Mar 11 '16 at 06:50

CrazyCasta

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Good general advice for `with` but this code pretty certainly does not run on anything else besides CPython :D – Antti Haapala -- Слава Україні Mar 11 '16 at 06:51
@AnttiHaapala I don't follow. It looks like pretty standard python that would at least run on pypy (I can't speak for Jython or IronPython as I don't know them well enough). Do you know something about the questioner's application that I do not? – CrazyCasta Mar 11 '16 at 07:08
it messes with CPython .pyc files, certainly wouldn't work on Jython which uses java .classes. At least Jython 2.7 seems to have `imp.get_magic` that returns the CPython get_magic, but marshal is not compatible. – Antti Haapala -- Слава Україні Mar 11 '16 at 07:11
Ah, marshal on jython can't marshal functions/code objects, so this definitely cannot run at all. – Antti Haapala -- Слава Україні Mar 11 '16 at 07:25
@AnttiHaapala I'm pretty sure that functions/code objects aren't marshal-able even by CPython. – CrazyCasta Mar 14 '16 at 04:53
You're very wrong there. The `.pyc` files are `marshal`led module contents. – Antti Haapala -- Слава Україні Mar 14 '16 at 07:28

Reverse engineering python 3

2 Answers2