Question

So I’m getting some interesting behaviour from some filters stacked within a for loop.
I’ll start with a demonstration:

>>> x = range(100)
>>> x = filter(lambda n: n % 2 == 0, x)
>>> x = filter(lambda n: n % 3 == 0, x)
>>> list(x)
[0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96]

Here we get the expected output. We have a range within a filter within a filter, and the filter conditions are stacking as we want them to. Now here comes my problem.
I have written a function for calculating the relative primes of a number. It looks like this:

def relative_primes(num):
    '''Returns a list of relative primes, relative to the given number.'''
    if num == 1:
        return []
    elif is_prime(num):
        return list(range(1, num))
    result = range(1, num)
    for factor in prime_factors(num):
        # Why aren't these filters stacking properly?                           
        result = filter(lambda n: n % factor != 0, result)
    return list(result)

For whatever reason, the filter is only being applied to the LAST factor in the list acquired from prime_factors(). Example:

>>> prime_factors(30)  
[2, 3, 5]  
>>> relative_primes(30)  
[1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 13, 14, 16, 17, 18, 19, 21, 22, 23, 24, 26, 27, 28, 29]

We can see that no multiples of 2 or 3 were removed from the list. Why is this happening? Why does the above example work, but the filters in the for loop don’t?

Answer 1

In Python 3.x, filter() returns a generator instead of a list. As such, only the final value of factor gets used since all three filters use the same factor. You will need to modify your lambda slightly in order to make it work.

result = filter(lambda n, factor=factor: n % factor != 0, result)