Question

I have some code like:

good = [x for x in mylist if x in goodvals]
bad = [x for x in mylist if x not in goodvals]

The goal is to split up the contents of mylist into two other lists, based on whether or not they meet a condition.

How can I do this more elegantly? Can I avoid doing two separate iterations over mylist? Can I improve performance by doing so?

Answer 1

good = [x for x in mylist if x in goodvals]
bad  = [x for x in mylist if x not in goodvals]

is there a more elegant way to do this?

That code is perfectly readable, and extremely clear!

# files looks like: [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ... ]
IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
images = [f for f in files if f[2].lower() in IMAGE_TYPES]
anims  = [f for f in files if f[2].lower() not in IMAGE_TYPES]

Again, this is fine!

There might be slight performance improvements using sets, but it’s a trivial difference, and I find the list comprehension far easier to read, and you don’t have to worry about the order being messed up, duplicates being removed as so on.

In fact, I may go another step "backward", and just use a simple for loop:

images, anims = [], []

for f in files:
    if f.lower() in IMAGE_TYPES:
        images.append(f)
    else:
        anims.append(f)

The a list-comprehension or using set() is fine until you need to add some other check or another bit of logic – say you want to remove all 0-byte jpeg’s, you just add something like..

if f[1] == 0:
    continue