Difference Between Two Lists with Duplicates in Python

Question

I have two lists that contain many of the same items, including duplicate items. I want to check which items in the first list are not in the second list. For example, I might have one list like this:

l1 = ['a', 'b', 'c', 'b', 'c']

and one list like this:

l2 = ['a', 'b', 'c', 'b']

Comparing these two lists I would want to return a third list like this:

l3 = ['c']

I am currently using some terrible code that I made a while ago that I'm fairly certain doesn't even work properly shown below.

def list_difference(l1,l2):
    for i in range(0, len(l1)):
        for j in range(0, len(l2)):
            if l1[i] == l1[j]:
                l1[i] = 'damn'
                l2[j] = 'damn'
    l3 = []
    for item in l1:
        if item!='damn':
            l3.append(item)
    return l3

How can I better accomplish this task?

Answer 1

You didn't specify if the order matters. If it does not, you can do this in >= Python 2.7:

l1 = ['a', 'b', 'c', 'b', 'c']
l2 = ['a', 'b', 'c', 'b']

from collections import Counter

c1 = Counter(l1)
c2 = Counter(l2)

diff = c1-c2
print list(diff.elements())