SHA-based subclass for random module

[Anton Vredegoor]

python at rcn.com (Raymond Hettinger) wrote:

>from random import Random
>from struct import unpack
>import md5
>
>class MD5Random(Random):
>    def random(self,    tofloat = 1.0 / 2 ** 53):
>        plaintxt = str(Random.getrandbits(self, 128))
>        ciphertxt = md5.new(plaintxt).digest()
>        randint = unpack('<Q', ciphertxt[:8])[0] >> 11  # grab 53 bits
>        return randint * tofloat


I wonder why it is necessary to do the bit twiddling once one has
chosen to multiply by a fraction instead of directly putting the bits
inside a float. If 1.0 / 2**53 is just some arbitrary value (and why
not: A floats' smallest value is a lot smaller than that?) then we can
generate integers of other sizes and divide them by other floats, for
example:

from random import Random
import md5

class MD5Random(Random):

    def random(self, tofloat = 1.0/2**128, prf = md5.new()):
        prf.update(repr(Random.random(self)))
        return int(prf.hexdigest(),16) * tofloat

def test():
    random = MD5Random().random
    for i in range(10):
        print random()
        
if __name__=='__main__':
    test()

Anton