Iterative deletion from list (Python 2)

Question

I've just started programming, and I'm solving Project Euler problems with Python for practice. (This is problem #2, finding the sum of the even fibonacci numbers within 4 million.) My problem appears in the loop at the bottom, where I'm trying to locate the odd numbers in the list, and delete them.

del fiblist[i] gives me the following error message:

Traceback (most recent call last): File ".../euler.py", line 35, in del fiblist[i] IndexError: list assignment index out of range

I don't see what I'm doing wrong here, and would really appreciate if anyone could help me see what I'm doing wrong here.

#euler2

def fibonacciList(limit):
    '''generates a list of fib numbers up to N'''
    mylist = []
    a,b = 1,2
    while True:
        if a <= limit:
            mylist.append(a)
            a,b = b,a+b
        else:
            break

    return mylist


fiblist = fibonacciList(4000000)

for i in fiblist:
    if i%2 != 0:    #if not even, delete from list
        print i
        del fiblist[i]

print fiblist

Answer 1

One problem here is that i is the item from the list, not it's index. So when you do del fiblist[i] you are not removing i, but the value that is at index i (which doesn't exist so you get an error). This can be fixed by using enumerate() to get indices to use, however, doing so introduces a new problem.

The main issue here is that you can't modify the length of a list while you iterate over it, as it messes with Python's iteration. One solution would be to copy the list and work on a copy, but the better one is to use a list comprehension to do what you want:

[i for i in fiblist if i%2 == 0]

This produces a new list with only the elements you want in. List comprehensions are a powerful tool, so I suggest you watch the video I linked for more.