Quick way to extend a set if we know elements are unique

You can use set.update to update your master set in place. This saves allocating a new set all the time so it should be a little faster than set.union…

>>> s = set(range(3))
>>> s.update(range(4))
>>> s
set([0, 1, 2, 3])

Of course, if you’re doing this in a loop:

masterSet = set()
for setA in iterable:
    masterSet = masterSet.union(setA)

You might get a performance boost by doing something like:

masterSet = set().union(*iterable)

Ultimately, membership testing of a set is O(1) (in the average case), so testing if the element is already contained in the set isn’t really a big performance hit.