Subtracting two lists in Python

Question

In Python, How can one subtract two non-unique, unordered lists? Say we have a = [0,1,2,1,0] and b = [0, 1, 1] I\'d like to do something like

Answer 1

Here's a relatively long but efficient and readable solution. It's O(n).

def list_diff(list1, list2):
    counts = {}
    for x in list1:
        try:
            counts[x] += 1
        except:
            counts[x] = 1
    for x in list2:
        try:
            counts[x] -= 1
            if counts[x] < 0:
                raise ValueError('All elements of list2 not in list2')
        except:
            raise ValueError('All elements of list2 not in list1') 
    result = []
    for k, v in counts.iteritems():
        result += v*[k] 
    return result

a = [0, 1, 1, 2, 0]
b = [0, 1, 1]
%timeit list_diff(a, b)
%timeit list_diff(1000*a, 1000*b)
%timeit list_diff(1000000*a, 1000000*b)
100000 loops, best of 3: 4.8 µs per loop
1000 loops, best of 3: 1.18 ms per loop
1 loops, best of 3: 1.21 s per loop