Error unpack requires a string argument of length 16

Question

Using Python 2.7 I had this code:

#Create a random 56 bit (7 byte) string
random_bytes = os.urandom(7)
random_bytes = int.from_bytes(random_bytes, byteorder="big")

Which gives the error:

AttributeError: type object 'int' has no attribute 'from_bytes'

After reading online it looks like from_bytes is Python 3. So I tried the following:

  random_bytes = os.urandom(7)
  #random_bytes = int.from_bytes(random_bytes, byteorder="big")
  random_bytes = struct.unpack('>16B', random_bytes)

But this gives the following error:

struct.error: unpack requires a string argument of length 16

Should the >16B be >7B? Even with that it seems to return a tuple.

The goal is to use random_bytes like this:

int(str(((time_bytes << 56) | random_bytes)) + code)

http://stackoverflow.com/questions/30402743/python-2-7-equivalent-of-built-in-method-int-from-bytes does this help? — timgeb, Feb 21 '16 at 18:45
You're asking struct to unpack a 7 byte sequence into a tuple of 16, 8 bit integers... — thebjorn, Feb 21 '16 at 18:50
Your goal statement doesn't need all those parenthesis or type conversions, it can be written simply as: `time_bytes << 56 | random_bytes + code` (it will be an int, and operator precedence is the same as your parens..) — thebjorn, Feb 21 '16 at 18:59

score 2 · Answer 1 · answered Feb 21 '16 at 18:54

2

Maybe this will help:

import os

binary = os.urandom(7)
result = int(binary.encode('hex'), 16)

answered Feb 21 '16 at 18:54

Cosinux

score 1 · Accepted Answer · answered Feb 21 '16 at 18:54

1

Maybe this will work for you:

r = chr(0) + os.urandom(7)  # pad with zeroes
struct.unpack('>q', r)

q being the code for signed 8-byte integer (Q is unsigned).

The result is a tuple with one element.

answered Feb 21 '16 at 18:54

thebjorn

2 Answers2