Reading a file until a match found with python

I have been trying this code to loop through randomly until a match found in the lookin.txt file and stop when a match is found with no success as my Python knowledge not that brilliant. Code is working only once but i need it to run continously until it finds a match. I appreciate if someone can direct me in the right direction.

        #! /usr/bin/env python
import random
randrange = random.SystemRandom().randrange
z = randrange( 1, 1000)

target_f = open("lookin.txt", 'rb')
read_f = target_f.read()
if z in read_f:
    file = open('found.txt', 'ab')
    file.write(z)
    print "Found ",z
else:
    print "Not found ",z

Lookin.txt:

453
7
8
56
78
332