Question

The python wiki says: “Membership testing with sets and dictionaries is much faster, O(1), than searching sequences, O(n). When testing “a in b”, b should be a set or dictionary instead of a list or tuple.”

I’ve been using sets in place of lists whenever speed is important in my code, but lately I’ve been wondering why sets are so much faster than lists. Could anyone explain, or point me to a source that would explain, what exactly is going on behind the scenes in python to make sets faster?

Answer 1

Sets are implemented using hash tables. Whenever you add an object to a set, the position within the memory of the set object is determined using the hash of the object to be added. When testing for membership, all that needs to be done is basically to look if the object is at the position determined by its hash, so the speed of this operation does not depend on the size of the set. For lists, in contrast, the whole list needs to be searched, which will become slower as the list grows.

This is also the reason that sets do not preserve the order of the objects you add.

Note that sets aren’t faster than lists in general — membership test is faster for sets, and so is removing an element. As long as you don’t need these operations, lists are often faster.