Python Sets Explained Simply (Unique and Unordered Collections)

Sets are collections that store unique items with no duplicates. They are useful for membership testing and removing duplicates quickly.

What is a set?

A set is created with curly braces {} and items separated by commas. Order does not matter.

fruits = {"apple", "banana", "cherry"}
numbers = {1, 2, 3, 4, 5}
mixed = {10, "hello", True}

You can also create a set from a list:

unique_numbers = set([1, 2, 2, 3, 3, 3])
print(unique_numbers)  # {1, 2, 3}

An empty set (note: {} makes a dictionary):

empty_set = set()

Sets automatically remove duplicates.

Adding and removing items

Add an item with add():

fruits.add("orange")
print(fruits)  # Includes orange

Remove an item with remove() (raises error if not found) or discard() (no error):

fruits.remove("banana")
fruits.discard("missing")  # No error

Common operations

Check membership:

print("apple" in fruits)  # True or False

Get length:

print(len(fruits))  # Number of unique items

Loop over a set:

for fruit in fruits:
    print(fruit)

Set operations

Union (all items):

set1 = {1, 2, 3}
set2 = {3, 4, 5}
combined = set1.union(set2)  # or set1 | set2
print(combined)  # {1, 2, 3, 4, 5}

Intersection (common items):

common = set1.intersection(set2)  # or set1 & set2
print(common)  # {3}

Difference (items in one but not both):

diff = set1.difference(set2)  # or set1 - set2
print(diff)  # {1, 2}

Simple examples

Remove duplicates from a list:

scores = [85, 92, 85, 78, 92]
unique_scores = set(scores)
print(unique_scores)  # {85, 92, 78}

Check common friends:

friends_a = {"Ali", "Sara", "Reza"}
friends_b = {"Sara", "Mohammad", "Reza"}
common = friends_a & friends_b
print(common)  # {"Sara", "Reza"}

Important notes

• Sets are unordered: items have no index.
• Sets are mutable, but items must be immutable (no lists inside sets).
• Sets are fast for checking if an item exists.

Quick summary

• Create sets with curly braces or set().
• Sets store unique items only.
• Use add(), remove(), and discard() to modify.
• Perform union, intersection, and difference easily.
• Ideal for removing duplicates and membership tests.

Practice using sets for unique collections. They provide efficient operations not available with lists in Python programs.