Convert a Python int into a big-endian string of bytes

Question

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

User · Answer

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?

User · Answer

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  amp  1     print int2bytes 1245427     - gt  b  x13 x00 xf3

User · Answer

You can use the struct module   import struct print struct pack   gt I   your int      gt I  is a format string   gt  means big endian and I means unsigned int  Check the documentation for more format chars

User · Answer

In Python 3 2   you can use int to bytes   If you don t want to specify the size   gt  gt  gt  n   1245427  gt  gt  gt  n to bytes  n bit length     7     8   big   or b  0  b  x13 x00 xf3    If you don t mind specifying the size   gt  gt  gt   1245427  to bytes 3  byteorder  big   b  x13 x00 xf3

User · Answer

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

User · Answer

This is fast and works for small and  arbitrary  large ints   def Dump n      s     x    n   if len s   amp  1      s    0    s   return s decode  hex   print repr Dump 1245427         x13 x00 xf3

User · Answer

Using the bitstring module    gt  gt  gt  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

User · Answer

The shortest way  I think  is the following   import struct val   0x11223344 val   struct unpack   lt I   struct pack   gt I   val   0  print   08x    val   This converts an integer to a byte-swapped integer

User · Answer

def tost i     result        while i      result append chr i amp 0xFF       i  gt  gt   8   result reverse     return    join result

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

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