Returns a list of elements that exist in both lists.

Create a set from a and b, then use the built-in set operator & to only keep values contained in both sets, then transform the set back into a list.

def intersection(a, b):
  _a, _b = set(a), set(b)
  return list(_a & _b)

intersection([1, 2, 3], [4, 3, 2]) # [2, 3]