Python Sets: Unique Elements and Fast Lookups

What Is a Set?

A set is an unordered collection of unique elements. Lookups run in O(1) — much faster than lists.

colours = {"red", "green", "blue"}
colours.add("red")     # no effect, already present
print(len(colours))    # 3

Removing Duplicates From a List

nums = [1, 2, 2, 3, 3, 4]
unique = list(set(nums))   # [1, 2, 3, 4]

Set Operations

a = {1, 2, 3}
b = {3, 4, 5}
print(a | b)   # union        {1,2,3,4,5}
print(a & b)   # intersection {3}
print(a - b)   # difference   {1,2}
print(a ^ b)   # symmetric    {1,2,4,5}

Membership Testing Is Fast

big = set(range(10_000_000))
print(9_999_999 in big)  # instant

When NOT to Use a Set

• Order matters → use a list.
• You need duplicates → use a list.
• Elements are mutable (e.g. lists) → sets require hashable items.