I have a few different lists that I want to turn into a set and use to find the difference from another set. Let's call them A, B, and C.

Is the more optimal way to do this set(A + B + C) or set(A).union(set(B)).union(set(C))

Would certain properties of A, B, and C like the number of duplicates or length affect this decision?

Would having an arbitrary number of sets?

For small lists, set(A + B + C) works fine. For larger lists, the following is more efficient because it does not create a temporary list:

myset = set(A)
myset.update(B)
myset.update(C)

A different approach uses itertools.chain, which is also efficient because it does not create temporary list:

import itertools
myset = set(itertools.chain(A, B, C))