The pure version of doubleStuff above made use of an important pattern for your toolbox. Whenever you need to write a function that creates and returns a list, the pattern is usually:
initialize a result variable to be an empty list loop create a new element append it to result return the result
Let us show another use of this pattern. Assume you already have a function is_prime(x) that can test if x is prime. Now, write a function to return a list of all prime numbers less than n:
def primes_upto(n): """ Return a list of all prime numbers less than n. """ result = [] for i in range(2, n): if is_prime(i): result.append(i) return result