How to filter a list [duplicate]

Question

Im writing a simple function to take out any odd numbers from a list and return a list of only the even ones.

def purify(numbers):
for i in numbers:
    if i%2!=0:
       numbers.remove(i)
return numbers    

print purify([4,5,5,4])

when applied above

it returns: [4, 5, 4] why doesnt the second 5 get removed as it also justifys the if?

Im looking less for a different method to the problem and more to understand why this happens.

thanks and sorry if this is stupid q.. Joe

Answer 1

When you remove an item, the items that follow get moved one position to the left. This results in the loop skipping some items.

BTW, a more idiomatic way to write that code is

numbers = [num for num in numbers if num % 2 == 0]

Answer 2

One option I didn't see mentioned was, ironically filter:

>>> filter(lambda x: not x % 2, [4,5,5,4])
[4, 4]