Convert python long/int to fixed size byte array

Everyone has overcomplicated this answer:

some_int = <256 bit integer>
some_bytes = some_int.to_bytes(32, sys.byteorder)
my_bytearray = bytearray(some_bytes)

You just need to know the number of bytes that you are trying to convert. In my use cases, normally I only use this large of numbers for crypto, and at that point I have to worry about modulus and what-not, so I don't think this is a big problem to be required to know the max number of bytes to return.

Since you are doing it as 768-bit math, then instead of 32 as the argument it would be 96.

I haven't done any benchmarks, but this recipe "works for me".

The short version: use '%x' % val, then unhexlify the result. The devil is in the details, though, as unhexlify requires an even number of hex digits, which %x doesn't guarantee. See the docstring, and the liberal inline comments for details.

from binascii import unhexlify

def long_to_bytes (val, endianness='big'):
    """
    Use :ref:`string formatting` and :func:`~binascii.unhexlify` to
    convert ``val``, a :func:`long`, to a byte :func:`str`.

    :param long val: The value to pack

    :param str endianness: The endianness of the result. ``'big'`` for
      big-endian, ``'little'`` for little-endian.

    If you want byte- and word-ordering to differ, you're on your own.

    Using :ref:`string formatting` lets us use Python's C innards.
    """

    # one (1) hex digit per four (4) bits
    width = val.bit_length()

    # unhexlify wants an even multiple of eight (8) bits, but we don't
    # want more digits than we need (hence the ternary-ish 'or')
    width += 8 - ((width % 8) or 8)

    # format width specifier: four (4) bits per hex digit
    fmt = '%%0%dx' % (width // 4)

    # prepend zero (0) to the width, to zero-pad the output
    s = unhexlify(fmt % val)

    if endianness == 'little':
        # see http://stackoverflow.com/a/931095/309233
        s = s[::-1]

    return s

...and my nosetest unit tests ;-)

class TestHelpers (object):
    def test_long_to_bytes_big_endian_small_even (self):
        s = long_to_bytes(0x42)
        assert s == '\x42'

        s = long_to_bytes(0xFF)
        assert s == '\xff'

    def test_long_to_bytes_big_endian_small_odd (self):
        s = long_to_bytes(0x1FF)
        assert s == '\x01\xff'

        s = long_to_bytes(0x201FF)
        assert s == '\x02\x01\xff'

    def test_long_to_bytes_big_endian_large_even (self):
        s = long_to_bytes(0xab23456c8901234567)
        assert s == '\xab\x23\x45\x6c\x89\x01\x23\x45\x67'

    def test_long_to_bytes_big_endian_large_odd (self):
        s = long_to_bytes(0x12345678901234567)
        assert s == '\x01\x23\x45\x67\x89\x01\x23\x45\x67'

    def test_long_to_bytes_little_endian_small_even (self):
        s = long_to_bytes(0x42, 'little')
        assert s == '\x42'

        s = long_to_bytes(0xFF, 'little')
        assert s == '\xff'

    def test_long_to_bytes_little_endian_small_odd (self):
        s = long_to_bytes(0x1FF, 'little')
        assert s == '\xff\x01'

        s = long_to_bytes(0x201FF, 'little')
        assert s == '\xff\x01\x02'

    def test_long_to_bytes_little_endian_large_even (self):
        s = long_to_bytes(0xab23456c8901234567, 'little')
        assert s == '\x67\x45\x23\x01\x89\x6c\x45\x23\xab'

    def test_long_to_bytes_little_endian_large_odd (self):
        s = long_to_bytes(0x12345678901234567, 'little')
        assert s == '\x67\x45\x23\x01\x89\x67\x45\x23\x01'

One-liner:

bytearray.fromhex('{:0192x}'.format(big_int))

The 192 is 768 / 4, because OP wanted 768-bit numbers and there are 4 bits in a hex digit. If you need a bigger bytearray use a format string with a higher number. Example:

>>> big_int = 911085911092802609795174074963333909087482261102921406113936886764014693975052768158290106460018649707059449553895568111944093294751504971131180816868149233377773327312327573120920667381269572962606994373889233844814776702037586419
>>> bytearray.fromhex('{:0192x}'.format(big_int))
bytearray(b'\x96;h^\xdbJ\x8f3obL\x9c\xc2\xb0-\x9e\xa4Sj-\xf6i\xc1\x9e\x97\x94\x85M\x1d\x93\x10\\\x81\xc2\x89\xcd\xe0a\xc0D\x81v\xdf\xed\xa9\xc1\x83p\xdbU\xf1\xd0\xfeR)\xce\x07\xdepM\x88\xcc\x7fv\\\x1c\x8di\x87N\x00\x8d\xa8\xbd[<\xdf\xaf\x13z:H\xed\xc2)\xa4\x1e\x0f\xa7\x92\xa7\xc6\x16\x86\xf1\xf3')
>>> lepi_int = 0x963b685edb4a8f336f624c9cc2b02d9ea4536a2df669c19e9794854d1d93105c81c289cde061c0448176dfeda9c18370db55f1d0fe5229ce07de704d88cc7f765c1c8d69874e008da8bd5b3cdfaf137a3a48edc229a41e0fa792a7c61686f1f
>>> bytearray.fromhex('{:0192x}'.format(lepi_int))
bytearray(b'\tc\xb6\x85\xed\xb4\xa8\xf36\xf6$\xc9\xcc+\x02\xd9\xeaE6\xa2\xdff\x9c\x19\xe9yHT\xd1\xd91\x05\xc8\x1c(\x9c\xde\x06\x1c\x04H\x17m\xfe\xda\x9c\x187\r\xb5_\x1d\x0f\xe5"\x9c\xe0}\xe7\x04\xd8\x8c\xc7\xf7e\xc1\xc8\xd6\x98t\xe0\x08\xda\x8b\xd5\xb3\xcd\xfa\xf17\xa3\xa4\x8e\xdc"\x9aA\xe0\xfay*|aho\x1f')

[My answer had used hex() before. I corrected it with format() in order to handle ints with odd-sized byte expressions. This fixes previous complaints about ValueError.]

With Python 3.2 and later, you can use int.to_bytes and int.from_bytes: https://docs.python.org/3/library/stdtypes.html#int.to_bytes