Find out if a number is prime. If it isn't, you'll get a list of factors. It's useful for finding GCM and other miscellaneous things about numbers.

For example, I was making a 1280x720 picture and I needed to know how it could be broken into squares.

$ ./isprime.py
Number: 1280
[2, 4, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320]
Number: 720
[2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 30, 36, 40, 45, 48, 60, 72, 80, 90, 120, 144, 180, 240]

Now I know it can be divided by 80 each way; and with a little division and then a little multiplication, 1280x720 can be made of 144 80x80 squares. Maybe you don't run into things like this every day, but I do :-)

#!/usr/bin/env python

def isprime(n):
    return [x for x in range(2, n / 2) if n % x == 0] or False

if __name__ == '__main__':
    while True:
        n = input('Number: ')
        print isprime(n)