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 c = a - b and have c be [2, 0] or [0, 2] order doesn't matter to me. This should throw an exception if a does not contain all elements in b.

Note this is different from sets! I'm not interested in finding the difference of the sets of elements in a and b, I'm interested in the difference between the actual collections of elements in a and b.

I can do this with a for loop, looking up the first element of b in a and then removing the element from b and from a, etc. But this doesn't appeal to me, it would be very inefficient (order of O(n^2) time) while it should be no problem to do this in O(n log n) time.

Answer 1

I know "for" is not what you want, but it's simple and clear:

for x in b:   a.remove(x) 

Or if members of b might not be in a then use:

for x in b:   if x in a:     a.remove(x) 

Answer 2

I would do it in an easier way:

a_b = [e for e in a if not e in b ] 

..as wich wrote, this is wrong - it works only if the items are unique in the lists. And if they are, it's better to use

a_b = list(set(a) - set(b))