Create a complement of list preserving duplicate values

Question

Given list a = [1, 2, 2, 3] and its sublist b = [1, 2] find a list complementing b in such a way that sorted(a) == sorted(b + complement)

Answer 1

If the order of elements in the complement doesn't matter, then collections.Counter is all that is needed:

from collections import Counter

a = [1, 2, 3, 2]
b = [1, 2]

complement = list((Counter(a) - Counter(b)).elements())  # complement = [2, 3]

If the order of items in the complement should be the same order as in the original list, then use something like this:

from collections import Counter, defaultdict
from itertools import count

a = [1,2,3,2]
b = [2,1]

c = Counter(b)
d = defaultdict(count)

complement = [x for x in a if next(d[x]) >= c[x]]  # complement = [3, 2]