How do I add the contents of an iterable to a set?

Just a quick update, timings using python 3:

#!/usr/local/bin python3
from timeit import Timer

a = set(range(1, 100000))
b = list(range(50000, 150000))

def one_by_one(s, l):
    for i in l:
        s.add(i)    

def cast_to_list_and_back(s, l):
    s = set(list(s) + l)

def update_set(s,l):
    s.update(l)

results are:

one_by_one 10.184448844986036
cast_to_list_and_back 7.969255169969983
update_set 2.212590195937082