[python] Convert a Python int into a big-endian string of bytes

I have a non-negative int and I would like to efficiently convert it to a big-endian string containing the same data. For example, the int 1245427 (which is 0x1300F3) should result in a string of length 3 containing three characters whose byte values are 0x13, 0x00, and 0xf3.

My ints are on the scale of 35 (base-10) digits.

How do I do this?

def tost(i):
  result = []
  while i:
    result.append(chr(i&0xFF))
    i >>= 8
  result.reverse()
  return ''.join(result)

You can use the struct module:

import struct
print struct.pack('>I', your_int)

'>I' is a format string. > means big endian and I means unsigned int. Check the documentation for more format chars.

Single-source Python 2/3 compatible version based on @pts' answer:

#!/usr/bin/env python
import binascii

def int2bytes(i):
    hex_string = '%x' % i
    n = len(hex_string)
    return binascii.unhexlify(hex_string.zfill(n + (n & 1)))

print(int2bytes(1245427))
# -> b'\x13\x00\xf3'

Probably the best way is via the built-in struct module:

>>> import struct
>>> x = 1245427
>>> struct.pack('>BH', x >> 16, x & 0xFFFF)
'\x13\x00\xf3'
>>> struct.pack('>L', x)[1:]  # could do it this way too
'\x13\x00\xf3'

Alternatively -- and I wouldn't usually recommend this, because it's mistake-prone -- you can do it "manually" by shifting and the chr() function:

>>> x = 1245427
>>> chr((x >> 16) & 0xFF) + chr((x >> 8) & 0xFF) + chr(x & 0xFF)
'\x13\x00\xf3'

Out of curiosity, why do you only want three bytes? Usually you'd pack such an integer into a full 32 bits (a C unsigned long), and use struct.pack('>L', 1245427) but skip the [1:] step?

The shortest way, I think, is the following:

import struct
val = 0x11223344
val = struct.unpack("<I", struct.pack(">I", val))[0]
print "%08x" % val

This converts an integer to a byte-swapped integer.

In Python 3.2+, you can use int.to_bytes:

If you don't want to specify the size

>>> n = 1245427
>>> n.to_bytes((n.bit_length() + 7) // 8, 'big') or b'\0'
b'\x13\x00\xf3'

If you don't mind specifying the size

>>> (1245427).to_bytes(3, byteorder='big')
b'\x13\x00\xf3'

Very easy with pwntools , the tools created for software hacking

(Un-ironically, I stumbled across this thread and tried solutions here, until I realised there exists conversion functionality in pwntools)

import pwntools

x2 = p32(x1)

Using the bitstring module:

>>> bitstring.BitArray(uint=1245427, length=24).bytes
'\x13\x00\xf3'

Note though that for this method you need to specify the length in bits of the bitstring you are creating.

Internally this is pretty much the same as Alex's answer, but the module has a lot of extra functionality available if you want to do more with your data.

This is fast and works for small and (arbitrary) large ints:

def Dump(n): 
  s = '%x' % n
  if len(s) & 1:
    s = '0' + s
  return s.decode('hex')
print repr(Dump(1245427))  #: '\x13\x00\xf3'