set

A set stores unique values only — duplicates are automatically dropped, and there's no guaranteed order.

JavaScript:

const numbers = new Set([1, 2, 2, 3]);

Python:

numbers = {1, 2, 2, 3}

print(numbers)   # {1, 2, 3}

Notice sets use curly braces { }, just like a dict — but with plain values instead of key-value pairs. (An empty {} in Python actually creates a dict, not a set — use set() for an empty set.)

Adding and removing

numbers = {1, 2, 3}

numbers.add(4)
print(numbers)   # {1, 2, 3, 4}

numbers.remove(2)
print(numbers)   # {1, 3, 4}

Same idea as JS's set.add() and set.delete().

Why use a set?

The most common reason: removing duplicates from a list.

fruits = ["apple", "banana", "apple", "cherry"]

unique_fruits = list(set(fruits))
print(unique_fruits)   # ["apple", "banana", "cherry"] (order not guaranteed)

Sets are also very fast for checking whether a value exists — you'll see this combined with the in keyword later in this phase.