Need help with python

hey /prog/
Need help with python
I am randomly generating all possible 20 character long strings consisting of 1234567890abcdef, and printing them to a file.
I generate the strings using:

CHARS="abcdef1234567890"
def rand_string(string_length):
        output = ''            
       
        for i in xrange(string_length):
                output += random.choice(CHARS)

Now i need to print rand_string(20) over and over again to an output file, but checking said file for that string, and only printing it if it isn't there yet.

Any thoughts?