Question

Here are two seemingly equivalent versions of a function for filtering out the primes from a list of numbers.

Version 1

def prime (mylist):
        for i in range(2, 8):
            return filter(lambda x: x == i or x % i, mylist)

Version 2

def prime2 (mylist):
    nums = mylist
    for i in range(2, 8): 
        nums = filter(lambda x: x == i or x % i, nums)
    return nums

print prime([2,3,4,5,6,7,8,9,10,11,12,13,14,15])  
   >> [2, 3, 5, 7, 9, 11, 13, 15]
print prime2([2,3,4,5,6,7,8,9,10,11,12,13,14,15]) 
   >> [2, 3, 5, 7, 11, 13]

Version 1 returns erroneous results. Why?

Answer 1

The first version only tests i == 2. In other words, it only tests whether 2 is a factor, instead of testing all integers from 2 to 7 as you intended. This is why it will (correctly) filter out all even numbers but will (wrongly) leave the odd ones which are not prime, like 9 and 15. Try this to see it explicitly:

def prime (mylist):
    for i in range(2, 8):
        print i # added to make things explicit; it's not necessary
        return filter(lambda x: x == i or x % i, mylist)

def prime2 (mylist):
    nums = mylist
    for i in range(2, 8):
        print i # added to make things explicit; it's not necessary
        nums = filter(lambda x: x == i or x % i, nums)
    return nums

print prime([2,3,4,5,6,7,8,9,10,11,12,13,14,15])
>>> 2
>>> [2, 3, 5, 7, 9, 11, 13, 15]
print prime2([2,3,4,5,6,7,8,9,10,11,12,13,14,15])
>>> 2
>>> 3
>>> 4
>>> 5
>>> 6
>>> 7
>>> [2, 3, 5, 7, 11, 13]