collatz.py

def collatz(n):
    yield n
    while True:
        if n == 1:
            return
        elif not n % 2:
            n = n / 2
        else:
            n = (3 * n) + 1
        yield n
        collatz(n)

def main():
    while True:
        try:
            n = int(raw_input("\n\nEnter a positive integer >"))
        except ValueError:
            print "Invalid input"
        else:
            break
    print ", ".join([str(x) for x in collatz(n)])

if __name__ == "__main__":
    main()