Sets

A set keeps only unique values. It's the quickest way to remove duplicates, or to check whether you've seen something before.

python

cities = ["Pune", "Delhi", "Pune", "Mumbai"]

unique = set(cities)

print(len(unique))

print("Delhi" in unique)

OUTPUT

3 True

Sets have no order, so you can't use an index like unique[0]. Checking x in some_set stays fast even with millions of values, which is why sets show up in dedup steps.