Reference a byte array as an integer

Question

In Python, I am aware certain methods exist in python 3 that can create a new integer from an existing byte array. However, I am looking for a way to create a reference to a byte array, as an integer. Such that, if the reference is changed, then the underlying byte array is also changed.

In C, this would be done like the following:

int main(void) {
  unsigned char bytes[4] = {1, 0, 0, 0};
  int* int_ref = (int*)bytes;
  *int_ref += 59;
  printf("bytes is now %u %u %u %u\n",
                        bytes[0],
                        bytes[1],
                        bytes[2],
                        bytes[3]);
  return 0;
}

The above program prints 60. I am looking for a way to do this in Python.

"However, I am looking for a way to create a reference to a byte array, as an integer." Such a thing does not exist in Python. If you want to work with stuff like that, use C. Python is a purely object-oriented language. Everything is an object. There are no pointers in the language either. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 26, 2018 at 22:11
Although, there may be something in the ctypes module to approximate what you are looking for, this screams XY problem to me. What is it that you are actually trying to accomplish? Why can't you just use the bytearray when you need an integer? Note, arr[i] will return an int type (which gets created on the fly). — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 26, 2018 at 22:17
@juanpa.arrivillaga I am trying to see if it's possible in python to make a byte code reader, that can read packed, typed data that's stored in binary. However, I am seeing the struct module may be more what i am looking for. I guess the reference idea might not be doable for python. — Josh Weinstein
– Josh Weinstein, Commented Dec 26, 2018 at 22:49
Actually, that's undefined behavior even in C. To do it legally you have to have an int object that is aliased by a char *, not the other way around. — Matteo Italia
– Matteo Italia, Commented Dec 26, 2018 at 22:54

martineau · Accepted Answer · 2018-12-26 23:00:15Z

1

Something along these lines seems close to what you want:

import _ctypes

def di(obj_id):
    """ Reverse of id() function. """
    # from https://stackoverflow.com/a/15012814/355230
    return _ctypes.PyObj_FromPtr(obj_id)

def func(obj_id):
    ba = di(obj_id)
    ba[0] += 50

data = bytearray([1, 0, 0, 0])
func(id(data))

print('bytes is now {} {} {} {}'.format(*data))  # -> bytes is now 51 0 0 0

answered Dec 26, 2018 at 23:00

martineau

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