[python] Convert python long/int to fixed size byte array

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'

Examples related to python

programming a servo thru a barometer Is there a way to view two blocks of code from the same file simultaneously in Sublime Text? python variable NameError Why my regexp for hyphenated words doesn't work? Comparing a variable with a string python not working when redirecting from bash script is it possible to add colors to python output? Get Public URL for File - Google Cloud Storage - App Engine (Python) Real time face detection OpenCV, Python xlrd.biffh.XLRDError: Excel xlsx file; not supported Could not load dynamic library 'cudart64_101.dll' on tensorflow CPU-only installation

Examples related to bytearray

How to create bitmap from byte array? Put byte array to JSON and vice versa Byte Array to Hex String Java ByteBuffer to String Fastest way to convert Image to Byte array How to store a byte array in Javascript Conversion from byte array to base64 and back How do I print bytes as hexadecimal? How to convert image into byte array and byte array to base64 String in android? How to convert a byte array to a hex string in Java?

Examples related to long-integer

"OverflowError: Python int too large to convert to C long" on windows but not mac How to check a Long for null in java What is the difference between "long", "long long", "long int", and "long long int" in C++? java.math.BigInteger cannot be cast to java.lang.Long A long bigger than Long.MAX_VALUE Definition of int64_t How to convert string to long Convert python long/int to fixed size byte array Converting Long to Date in Java returns 1970 Division of integers in Java

Examples related to diffie-hellman

Convert python long/int to fixed size byte array Why does SSL handshake give 'Could not generate DH keypair' exception?

Examples related to rc4-cipher

Convert python long/int to fixed size byte array