first_list = [(1, 'a'), (2, 'b'), (3, 'c')]
second_list = [1, 2, 3, 4, 4, 5]
set(second_list) - set(x[0] for x in first_list)
set(second_list) - set(map(f, first_list))
Referring to the first element of all tuples in a list of tuples