Sets

Sets

What it means

A set ({1, 2, 3}) is an unordered collection of unique, hashable items — duplicates are automatically discarded. Sets are optimized for fast membership testing (in) and for mathematical set operations like union, intersection, and difference.

Examples

fruits = {"apple", "banana", "apple"}   # {'apple', 'banana'} -- duplicate dropped

fruits.add("cherry")
fruits.discard("banana")               # remove if present, no error if missing

a = {1, 2, 3}
b = {2, 3, 4}

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

# Fast membership test
print(3 in a)   # True -- O(1) average, unlike checking a list

Common mistake

Trying to create an empty set with {} — that actually creates an empty dict, not a set, because {} is the dict literal.

empty = {}
print(type(empty))          # <class 'dict'>  <- not a set!

empty_set = set()
print(type(empty_set))      # <class 'set'>   <- correct way

Quick practice

1. What happens when you add a duplicate item to a set?

   Answer

   Nothing changes — sets silently ignore additions of items that already exist.

2. How do you create an empty set?

   Answer

   set() — not {}, which creates an empty dict instead.

3. Why is checking x in my_set generally faster than x in my_list?

   Answer

   Sets use a hash table internally, giving average O(1) membership checks, while lists require a linear O(n) scan.