Sets in Python — Set Operations and Methods

What is a Set?

A set is an unordered collection of unique items. Sets automatically remove duplicates and are highly optimized for membership testing.

Unordered - no guaranteed order
Unique items only - duplicates are removed
Mutable - you can add/remove items
O(1) average lookup - very fast membership test
No indexing or slicing

Creating Sets

empty = set()           # NOT {} - that creates an empty dict!
fruits = {"apple", "banana", "mango"}
nums = {1, 2, 3, 2, 1}  # duplicates removed
print(nums)             # {1, 2, 3}

# From other iterables
from_list = set([1, 2, 2, 3, 3, 4])
from_str  = set("hello")   # {'h', 'e', 'l', 'o'}
from_range = set(range(5)) # {0, 1, 2, 3, 4}

print(type(fruits))  # <class 'set'>
print(len(fruits))   # 3

Adding & Removing Items

Modifying Sets

fruits = {"apple", "banana", "mango"}

# Add
fruits.add("orange")
fruits.update(["grape", "kiwi"])  # add multiple

# Remove
fruits.remove("banana")    # raises KeyError if not found
fruits.discard("papaya")   # safe - no error if not found
popped = fruits.pop()      # removes and returns a random item
fruits.clear()             # removes all items

# Check membership
colors = {"red", "green", "blue"}
print("red" in colors)     # True
print("yellow" in colors)  # False

Set Operations (Math)

Sets support mathematical operations like union, intersection, and difference.

Set Operations

a = {1, 2, 3, 4, 5}
b = {4, 5, 6, 7, 8}

# Union - all items from both sets
print(a | b)           # {1, 2, 3, 4, 5, 6, 7, 8}
print(a.union(b))      # same

# Intersection - items in both sets
print(a & b)                  # {4, 5}
print(a.intersection(b))      # same

# Difference - items in a but not b
print(a - b)                  # {1, 2, 3}
print(a.difference(b))        # same

# Symmetric difference - items in either but not both
print(a ^ b)                          # {1, 2, 3, 6, 7, 8}
print(a.symmetric_difference(b))      # same

# Subset and superset
x = {1, 2}
print(x.issubset(a))    # True  - all of x is in a
print(a.issuperset(x))  # True  - a contains all of x
print(a.isdisjoint({9, 10}))  # True - no common items

Set Methods Reference

Method	Description
`add(x)`	Add element x
`update(iterable)`	Add multiple elements
`remove(x)`	Remove x (KeyError if missing)
`discard(x)`	Remove x (no error if missing)
`pop()`	Remove and return a random element
`clear()`	Remove all elements
`union(s)`	Return union of sets
`intersection(s)`	Return common elements
`difference(s)`	Return elements not in s
`symmetric_difference(s)`	Return elements in either but not both
`issubset(s)`	True if all elements are in s
`issuperset(s)`	True if s is a subset
`isdisjoint(s)`	True if no common elements
`copy()`	Return a shallow copy

frozenset - Immutable Set

frozenset

# frozenset is immutable - can be used as dict key
fs = frozenset([1, 2, 3])
print(fs)  # frozenset({1, 2, 3})

# Can be used as a dict key (regular set cannot)
lookup = {frozenset([1, 2]): "pair", frozenset([3]): "single"}

# Practical: remove duplicates from a list while preserving order
def unique(lst):
    seen = set()
    return [x for x in lst if not (x in seen or seen.add(x))]

data = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3]
print(unique(data))  # [3, 1, 4, 5, 9, 2, 6]