Python/maths/binary_exp_mod.py

def bin_exp_mod(a, n, b):
    """
    >>> bin_exp_mod(3, 4, 5)
    1
    >>> bin_exp_mod(7, 13, 10)
    7
    """
    # mod b
    assert b != 0, "This cannot accept modulo that is == 0"
    if n == 0:
        return 1

    if n % 2 == 1:
        return (bin_exp_mod(a, n - 1, b) * a) % b

    r = bin_exp_mod(a, n / 2, b)
    return (r * r) % b


if __name__ == "__main__":
    try:
        BASE = int(input("Enter Base : ").strip())
        POWER = int(input("Enter Power : ").strip())
        MODULO = int(input("Enter Modulo : ").strip())
    except ValueError:
        print("Invalid literal for integer")

    print(bin_exp_mod(BASE, POWER, MODULO))