true random number generator based (d20) dice roller, verified fair

trng_r1dx.py

f = open('/dev/urandom', 'rb')
default_n = 1000000
max_r = 256
def r1dx(x=20):
    while True:
        # get one byte, take as int on [1,256]
        r = int.from_bytes(f.read(1), 'little')+1
        # if byte is less than the max factor of 'x' on the interval max_r, return r%x+1
        if r < (max_r-(max_r%x)+1): return (r%x)+1

if __name__ == "__main__":
    import itertools
    # get N sorted values taken from r1dx()
    def sample(n=default_n): return sorted([r1dx() for _ in range(n)])
    # run-length encoding
    def rle(xs): return [(len(list(gp)), x) for x, gp in itertools.groupby(xs)]
    # print a table of the decimal rate of occurrence for each value
    print('\n'.join(["%.4f %d" % (x[0]/default_n, x[1]) for x in rle(sample())]))