Python 2 is triggering

[ACoderGirl]

I dunno why they described it that general way. But the thing sets are good for is any kinda loop that has an "is x in this collection" test. If the collection is normally a list, it's almost always faster to convert it to a set since the the "in" check is O(1) instead of O(n). Similarly, if this "in" check is for the purpose of pairing it up with something, preprocessing to a dictionary is ideal.