Get difference from two lists in Python

Question

I have two lists, l1 and l2. I need items from l1 which are not in l2.

l1 = [2, 3, 4, 5]
l2 = [0, 1, 2, 3]

I want to get only [4,5] - only new values in l1.

[i for i in l1 if not i in l2 ]

Can I do that without iteration?

Answer 1

Short answer, yes: list(set(l1) - set(l2)), though this will not keep order.

Long answer, no, since internally the CPU will always iterate. Though if you use set() that iteration will be done highly optimized and will be much faster then your list comprehension (not to mention that checking for membership value in list is much faster with sets then lists).

Answer 2

You can't do it without iteration. Even if you call a single method, internally that will iterate.

Your approach is fine for a small list, but you could use this approach instead for larger lists:

s2 = set(l2)
result = [i for i in l1 if not i in s2 ]

This will be fast and will also preserve the original order of the elements in l1.