Chapter 21: Sets 🎲¶

🚀 Open Notebook¶

📺 Video Tutorial¶

Python lists, sets, and tuples explained 🍍 (13:04)

What You’ll Learn¶

Master Python sets, an unordered collection type that automatically handles unique values and provides powerful mathematical set operations.

Learning Objectives¶

By the end of this chapter, you will:

Understand what sets are and when to use them
Create and manipulate sets using methods
Leverage automatic duplicate removal
Apply set operations (union, intersection, difference)
Choose between lists, sets, and tuples appropriately
Optimize code using set membership testing

Concept Explanation¶

What is a Set?¶

A set is an unordered collection of unique items. Think of it like a bag of items where:

No duplicates allowed
No specific order (can’t access by index)
Items are immutable (but you can add/remove from set)
Very fast membership testing

Collection Types Comparison¶

Type	Symbol	Ordered	Changeable	Duplicates	Speed
List	`[]`	✅ Yes	✅ Yes	✅ Yes	Medium
Set	`{}`	❌ No	✅ Yes*	❌ No	Fast
Tuple	`()`	✅ Yes	❌ No	✅ Yes	Fastest

*Can add/remove items, but items themselves must be immutable

Set Syntax¶

# Creating sets
empty_set = set()  # Must use set(), not {}
fruits = {"apple", "banana", "cherry"}
numbers = {1, 2, 3, 4, 5}
mixed = {1, "hello", 3.14}  # Different types OK

Why Use Sets?¶

Remove duplicates automatically
Fast membership testing (in operator)
Mathematical operations (union, intersection)
Unique collections (tags, categories, IDs)

Examples¶

Example 1: Creating Sets¶

# Basic set creation
colors = {"red", "green", "blue"}

# From list (removes duplicates)
numbers = set([1, 2, 2, 3, 3, 3])
print(numbers)  # {1, 2, 3}

# Empty set (careful!)
wrong = {}  # This is a dictionary!
correct = set()  # This is an empty set

Example 2: Automatic Duplicate Removal¶

# Duplicates automatically removed
fruits = {"apple", "banana", "apple", "cherry"}
print(fruits)  # {'apple', 'banana', 'cherry'}

# Remove duplicates from list
numbers = [1, 2, 2, 3, 3, 3, 4]
unique = set(numbers)
print(unique)  # {1, 2, 3, 4}

Example 3: Adding and Removing¶

fruits = {"apple", "banana"}

# Add single item
fruits.add("cherry")  # {'apple', 'banana', 'cherry'}

# Try to add duplicate (no effect)
fruits.add("apple")  # Still {'apple', 'banana', 'cherry'}

# Remove item
fruits.remove("banana")  # {'apple', 'cherry'}

# Safe remove (doesn't error if missing)
fruits.discard("orange")  # No error even though orange not in set

Example 4: Set Operations¶

set_a = {1, 2, 3, 4}
set_b = {3, 4, 5, 6}

# Union (all items from both)
print(set_a | set_b)  # {1, 2, 3, 4, 5, 6}

# Intersection (items in both)
print(set_a & set_b)  # {3, 4}

# Difference (in A but not B)
print(set_a - set_b)  # {1, 2}

# Symmetric difference (in either, not both)
print(set_a ^ set_b)  # {1, 2, 5, 6}

Example 5: Membership Testing¶

# Fast membership check
valid_users = {"alice", "bob", "charlie"}

user = "alice"
if user in valid_users:
    print(f"Welcome, {user}!")  # Fast lookup!

Example 6: Iterating Sets¶

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

# Order may vary each run!
for fruit in fruits:
    print(fruit)

Example 7: Set Comprehension¶

# Create set of squares
squares = {x**2 for x in range(10)}
print(squares)  # {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}

# Filter vowels from string
text = "hello world"
vowels = {char for char in text if char in "aeiou"}
print(vowels)  # {'o', 'e'}

Practice Exercises¶

Beginner¶

Create Set: Make a set of your 5 favorite foods
Remove Duplicates: Convert list [1,1,2,2,3,3] to unique values
Add Items: Start with empty set, add 5 numbers
Membership Test: Check if “python” is in set of languages
Count Unique: Count unique letters in “mississippi”

Intermediate¶

Common Items: Find common elements in two sets of numbers
Unique Words: Find all unique words in a sentence
Set Difference: Find items in set A but not in set B
Symmetric Diff: Find items in either set but not both
Update Set: Use update() to add multiple items at once

Advanced¶

Tag System: Build a tagging system with set operations
Duplicate Finder: Find duplicate items across multiple lists
Set Math: Implement set operations manually without operators
Frozen Sets: Create immutable sets and understand use cases
Optimize Search: Replace list membership with set for performance

Common Mistakes to Avoid¶

❌ Mistake 1: Empty Set Syntax¶

# WRONG: Creates dictionary, not set!
empty = {}
print(type(empty))  # <class 'dict'>

# CORRECT: Use set() function
empty = set()
print(type(empty))  # <class 'set'>

❌ Mistake 2: Expecting Order¶

# WRONG: Assuming sets maintain order
numbers = {5, 2, 8, 1, 9}
print(numbers)  # Order not guaranteed!

# CORRECT: Use list if order matters
numbers = [5, 2, 8, 1, 9]

❌ Mistake 3: Trying to Index¶

fruits = {"apple", "banana", "cherry"}
# WRONG: Sets don't support indexing
# print(fruits[0])  # TypeError!

# CORRECT: Convert to list if needed
fruits_list = list(fruits)
print(fruits_list[0])

❌ Mistake 4: Mutable Items in Set¶

# WRONG: Lists are mutable, can't be in sets
# my_set = {[1, 2], [3, 4]}  # TypeError!

# CORRECT: Use tuples (immutable)
my_set = {(1, 2), (3, 4)}  # Works!

Real-World Applications¶

1. Remove Duplicate Emails¶

# Email list with duplicates
emails = ["user@example.com", "admin@site.com", "user@example.com"]

# Get unique emails
unique_emails = set(emails)
print(f"Unique subscribers: {len(unique_emails)}")

2. Tag System¶

post1_tags = {"python", "programming", "tutorial"}
post2_tags = {"python", "web", "flask"}

# Find common tags
common = post1_tags & post2_tags
print(f"Common tags: {common}")  # {'python'}

3. Permissions System¶

user_permissions = {"read", "write"}
required_permissions = {"read", "write", "delete"}

# Check if user has all required permissions
if required_permissions.issubset(user_permissions):
    print("Access granted")
else:
    missing = required_permissions - user_permissions
    print(f"Missing permissions: {missing}")

4. Data Validation¶

valid_statuses = {"pending", "approved", "rejected"}

def validate_status(status):
    return status.lower() in valid_statuses

print(validate_status("APPROVED"))  # True (fast lookup!)

Challenge Projects¶

Project 1: Student Enrollment System¶

Create a system that:

Tracks students enrolled in courses using sets
Finds students in multiple courses
Identifies courses unique to each semester
Calculates total unique students

Project 3: Inventory Deduplicator¶

Develop a program that:

Removes duplicate SKUs from inventory
Finds items in warehouse A but not B
Merges inventory from multiple sources
Reports unique vs duplicate items

Project 4: Word Analyzer¶

Create a tool that:

Finds unique words in documents
Compares vocabulary between texts
Identifies unique words per author
Builds word frequency analysis

Project 5: Access Control System¶

Build a permissions manager:

Define role-based permissions as sets
Check user access levels
Combine permissions from multiple roles
Audit permission differences

🎓 Key Takeaways from Video¶

Variables store data values that can be reused
Use loops to repeat actions
Follow along with the video for hands-on practice

💡 These points cover the main concepts from the video tutorial to help reinforce your learning.