Demonstration of `int.from_bytes()`

bytes_to_int.py

# Demonstration of how `int.from_bytes()` converts a Python bytes obj to int.
# Used in random module's `SystemRandom.random()` to generate a float in [0.0, 1.0) from `os.urandom()`
# https://github.com/python/cpython/blob/c6040638aa1537709add895d24cdbbb9ee310fde/Lib/random.py#L676
#
# An example: the number 1984 can be "decomposed" in the decimal system (base 10)
# as (1 * 10 ** 3)
#  + (9 * 10 ** 2)
#  + (8 * 10 ** 1)
#  + (4 * 10 ** 0)
#
# `int.from_bytes()` (with big endianness) uses a base-256 numbering system
# Given b = b'\xe6\x04\x00\x00', 
# >>> list(b)
# [230, 4, 0, 0]
# And the int is then
#    (230 * 256 ** 3)
#  + (4   * 256 ** 2)
#  + (0   * 256 ** 1)
#  + (0   * 256 ** 0) == 3859021824


def bytes_to_int(b: bytes, byteorder: str='big') -> int:
    if byteorder == 'big':
        return sum(j * 256 ** i for i, j in enumerate(b[::-1]))
    elif byteorder == 'little':
        return sum(j * 256 ** i for i, j in enumerate(b))
    else:
        raise ValueError("byteorder must be either 'little' or 'big'.")


def test(seed=None):
    for _ in range(10):
        b = os.urandom(random.randint(0, 100))
        assert bytes_to_int(b, 'big') == int.from_bytes(b, 'big'), b
        assert bytes_to_int(b, 'little') == int.from_bytes(b, 'little'), b


if __name__ == '__main__'
    import os
    import random
    import sys
    
    sys.exit(test())