None or no argument seeds from current time or from an operating
system specific randomness source if available.

if a is None:
    try:
        a = long(_hexlify(_urandom(16)), 16)
    except NotImplementedError:
        import time
        a = long(time.time() * 256) # use fractional seconds

random = self.random
u = random()
while u <= 1e-7:
    u = random()
return -_log(u)/lambd