Sets

Set Basics

A set is an unordered collection of unique, immutable elements enclosed in curly braces {}:

fruits = {"apple", "banana", "cherry"}
numbers = {1, 2, 3, 2, 1}   # {1, 2, 3} — duplicates removed
empty_set = set()            # not {} — that's a dict!
mixed = {1, "hello", (1, 2)}  # must be hashable elements

Adding and Removing Elements

s = {1, 2, 3}

s.add(4)          # {1, 2, 3, 4}
s.update([5, 6])  # {1, 2, 3, 4, 5, 6}
s.add(1)          # no change (already exists)

s.remove(3)       # {1, 2, 4, 5, 6}
# s.remove(99)    # KeyError!
s.discard(99)     # no error (safe remove)
popped = s.pop()  # removes and returns arbitrary element
s.clear()         # set()

Set Operations

Sets support mathematical set operations:

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

a | b           # union: {1, 2, 3, 4, 5, 6}
a.union(b)      # same

a & b           # intersection: {3, 4}
a.intersection(b)  # same

a - b           # difference: {1, 2}
a.difference(b) # same

a ^ b           # symmetric difference: {1, 2, 5, 6}
a.symmetric_difference(b)  # same

Comparison Operators

{1, 2}.issubset({1, 2, 3})   # True
{1, 2} <= {1, 2, 3}         # True (subset)
{1, 2, 3}.issuperset({1, 2})  # True
{1, 2, 3} >= {1, 2}         # True (superset)
{1, 2}.isdisjoint({3, 4})   # True (no common elements)

Useful Set Methods

s = {3, 1, 4, 1, 5}
len(s)           # 4 (duplicates removed)
1 in s           # True
99 not in s      # True
copy = s.copy()  # shallow copy

Set Comprehension

squares = {x ** 2 for x in range(10)}
evens = {x for x in range(20) if x % 2 == 0}
print(squares)  # {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}

Frozenset

An immutable version of a set:

fs = frozenset([1, 2, 3, 2, 1])
# fs.add(4)  # AttributeError!
# fs.remove(1)  # AttributeError!

# Useful as dict keys or in other sets
sets_set = {frozenset([1, 2]), frozenset([3, 4])}

Common Use Cases

# Remove duplicates from a list
items = [1, 2, 3, 2, 1, 4, 3]
unique = list(set(items))  # [1, 2, 3, 4]

# Find common elements
friends_a = {"Alice", "Bob", "Charlie"}
friends_b = {"Bob", "Diana", "Eve"}
common = friends_a & friends_b  # {"Bob"}

# Check if all items in one list are in another
required = {"name", "email", "age"}
provided = {"name", "email"}
missing = required - provided  # {"age"}