Python: Miller-Rabin Primality Test

import random


def is_prime(n, k=5):
    if n < 6:
        return [False, False, True, True, False, True][n]
    elif n & 1 == 0:
        return False
    else:
        s, d = 0, n - 1
        while d & 1 == 0:
            s, d = s + 1, d >> 1
        for a in random.sample(xrange(2, min(n - 2, sys.maxint)), min(n - 4, k)):
            x = pow(a, d, n)
            if x != 1 and x + 1 != n:
                for r in xrange(1, s):
                    x = pow(x, 2, n)
                    if x == 1:
                        return False
                    elif x == n - 1:
                        a = 0
                        break
                if a:
                    return False
        return True