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Section 10.22 Functions that Produce Lists

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
   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):
    return result
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