Sets: each item only once

A set keeps each item only once

A set is a collection of unique items. Unique means each item can appear only once. If you add a duplicate, the set simply ignores it. You write a set with curly braces { }, like a dict, but with single items instead of key: value pairs.

cities = {"Pune", "Delhi", "Pune", "Chennai"}
print(len(cities))
print("Delhi" in cities)

Removing duplicates from a list

phones = ["98450", "98111", "98450", "99000", "98111"]
unique = set(phones)
print(len(unique))
print(sorted(unique))

Adding and removing

present = {"Asha", "Ravi"}
present.add("Meena")
present.add("Asha")
present.discard("Ravi")
present.discard("John")
print(sorted(present))

There is also remove(), which works like discard but gives KeyError when the item is missing. discard is the safer choice when you are not sure.

Why sets are fast

To check x in my_list, Python looks at the items one by one, which is slow for a lakh of items. A set stores items using a method called hashing, which turns each item into a number that says where it is kept. So x in my_set is almost instant, even for millions of items. If you check membership many times, turn the list into a set first.

Set maths: students in two clubs

cricket = {"Asha", "Ravi", "Meena", "Arjun"}
dance = {"Meena", "Priya", "Arjun"}
print(sorted(cricket | dance))
print(sorted(cricket & dance))
print(sorted(cricket - dance))

a = {}
b = set()
print(type(a))
print(type(b))