Chapter 8: Dictionaries: Key-Value Pairs 📖

In the last chapter, you learned about lists — collections where you access items by their position (index 0, 1, 2…). But what if you want to organize data using meaningful labels instead of numbers? What if you want to store a person’s name, age, and email together in a way that makes sense?

That’s where dictionaries come in! They let you store data using descriptive keys like "name" or "age" instead of remembering that index 0 is the name and index 1 is the age. This makes your code more readable and powerful.

Introduction to Dictionaries 🎯

A dictionary is a collection of key-value pairs. Each key acts like a label that points to a value. Think of it like a real dictionary where you look up a word (the key) to find its definition (the value).

Creating Dictionaries

To create a dictionary, use curly braces {} with key-value pairs separated by colons:

# A simple dictionary
student = {
    "name": "Jordan",
    "age": 14,
    "grade": "9th"
}

Each entry has:

A key (like "name", "age", "grade")
A colon :
A value (like "Jordan", 14, "9th")
A comma separating each pair (except the last one)

Keys are usually strings, but values can be any data type — strings, numbers, booleans, lists, or even other dictionaries!

More Examples

# Game character stats
character = {
    "name": "DragonSlayer",
    "health": 100,
    "mana": 50,
    "level": 12,
    "equipped": True
}

# Product information
product = {
    "name": "Gaming Mouse",
    "price": 49.99,
    "in_stock": True,
    "quantity": 15
}

# Empty dictionary
empty_dict = {}

Why Use Dictionaries?

Compare these two approaches:

Using a list (confusing):

student = ["Jordan", 14, "9th", "Math"]
# Is index 0 the name or age? Hard to remember!
print(student[0])  # What does this represent?

Using a dictionary (clear):

student = {
    "name": "Jordan",
    "age": 14,
    "grade": "9th",
    "favorite_subject": "Math"
}
print(student["name"])  # Obviously the name!

Dictionaries make your code self-documenting and much easier to understand.

Displaying Dictionaries

You can print an entire dictionary:

player = {"name": "Alex", "score": 1250, "level": 5}
print(player)
# Shows: {'name': 'Alex', 'score': 1250, 'level': 5}

Accessing Dictionary Values 🔍

Unlike lists where you use numeric indices, dictionaries use keys to access values. This makes your code much more readable.

Using Keys

To access a value, use square brackets with the key:

student = {
    "name": "Jordan",
    "age": 14,
    "grade": "9th",
    "gpa": 3.8
}

print(student["name"])   # Shows: Jordan
print(student["age"])    # Shows: 14
print(student["gpa"])    # Shows: 3.8

Keys Are Case-Sensitive

Make sure your key matches exactly:

player = {"name": "Alex", "score": 1250}

print(player["name"])   # Works!
print(player["Name"])   # ERROR: KeyError: 'Name'

"name" and "Name" are different keys.

Using `.get()` Method

If you try to access a key that doesn’t exist, Python raises a KeyError:

student = {"name": "Jordan", "age": 14}
print(student["email"])  # ERROR: KeyError: 'email'

The .get() method provides a safer alternative. It returns None if the key doesn’t exist (or a default value you specify):

student = {"name": "Jordan", "age": 14}

# Returns None if key doesn't exist
email = student.get("email")
print(email)  # Shows: None

# Return a default value if key doesn't exist
email = student.get("email", "No email on file")
print(email)  # Shows: No email on file

# Key exists - works normally
name = student.get("name")
print(name)  # Shows: Jordan

This is especially useful when you’re not sure if a key exists.

Using Dictionary Values

Once you access a value, you can use it like any other variable:

product = {
    "name": "Laptop",
    "price": 999.99,
    "tax_rate": 0.08
}

price = product["price"]
tax = price * product["tax_rate"]
total = price + tax

print(f"Product: {product['name']}")
print(f"Price: ${price:.2f}")
print(f"Tax: ${tax:.2f}")
print(f"Total: ${total:.2f}")

Checking if a Key Exists

Use the in operator to check if a key exists before accessing it:

student = {"name": "Jordan", "age": 14, "grade": "9th"}

if "email" in student:
    print(student["email"])
else:
    print("Email not found")

if "name" in student:
    print(f"Student name: {student['name']}")

Adding and Updating Entries ✏️

Dictionaries are mutable, meaning you can change them after creation. You can add new key-value pairs or update existing ones.

Adding New Entries

To add a new key-value pair, simply assign a value to a new key:

student = {
    "name": "Jordan",
    "age": 14
}

print(student)  # Shows: {'name': 'Jordan', 'age': 14}

# Add new entries
student["grade"] = "9th"
student["gpa"] = 3.8
student["email"] = "jordan@example.com"

print(student)
# Shows: {'name': 'Jordan', 'age': 14, 'grade': '9th', 'gpa': 3.8, 'email': 'jordan@example.com'}

If the key already exists, it updates the value. If it doesn’t, it creates a new entry.

Updating Existing Entries

To update a value, assign a new value to an existing key:

player = {
    "name": "DragonSlayer",
    "health": 100,
    "score": 1000
}

print(f"Initial score: {player['score']}")

# Take damage
player["health"] -= 25
print(f"Health after damage: {player['health']}")

# Earn points
player["score"] += 500
print(f"New score: {player['score']}")

Using `.update()` Method

The .update() method adds or updates multiple entries at once:

student = {
    "name": "Jordan",
    "age": 14
}

# Update multiple fields
student.update({
    "age": 15,           # Update existing
    "grade": "10th",     # Add new
    "honor_roll": True   # Add new
})

print(student)

This is convenient when you have several updates to make.

Practical Example: Character Stats

# Initialize character
character = {
    "name": "Warrior",
    "level": 1,
    "health": 100,
    "mana": 50,
    "experience": 0
}

print("=== Character Created ===")
print(f"Name: {character['name']}")
print(f"Level: {character['level']}")
print()

# Character gains experience
print("Character completes a quest!")
character["experience"] += 150

# Level up when experience reaches 100
if character["experience"] >= 100:
    character["level"] += 1
    character["health"] += 20
    character["mana"] += 10
    print("🎉 Level Up!")

print(f"Level: {character['level']}")
print(f"Health: {character['health']}")
print(f"Experience: {character['experience']}")

Removing Entries 🗑️

Python provides several ways to remove entries from dictionaries.

Using `del` Statement

Remove a key-value pair by key:

student = {
    "name": "Jordan",
    "age": 14,
    "temp_id": "TMP123",
    "grade": "9th"
}

del student["temp_id"]  # Remove the temporary ID
print(student)
# Shows: {'name': 'Jordan', 'age': 14, 'grade': '9th'}

Using `.pop()` Method

The .pop() method removes a key and returns its value:

inventory = {
    "sword": 1,
    "shield": 1,
    "potion": 5
}

# Use a potion
potions_left = inventory.pop("potion")
print(f"Used 1 potion, {potions_left} total")
inventory["potion"] = potions_left - 1

print(inventory)

You can provide a default value if the key might not exist:

score = game_data.pop("bonus", 0)  # Returns 0 if "bonus" doesn't exist

Using `.clear()` Method

Remove all entries but keep the dictionary:

cart = {"item1": "Book", "item2": "Pen", "item3": "Notebook"}
cart.clear()
print(cart)  # Shows: {}

Looping Through Dictionaries 🔁

Dictionaries are incredibly powerful when combined with loops. You can iterate over keys, values, or both.

Looping Over Keys

By default, looping over a dictionary gives you the keys:

grades = {
    "Math": 90,
    "Science": 85,
    "English": 92,
    "History": 88
}

for subject in grades:
    print(subject)

Output:

Math
Science
English
History

You can also explicitly use .keys():

for subject in grades.keys():
    print(subject)

Looping Over Values

To get just the values, use .values():

grades = {
    "Math": 90,
    "Science": 85,
    "English": 92,
    "History": 88
}

for grade in grades.values():
    print(f"Grade: {grade}")

Output:

Grade: 90
Grade: 85
Grade: 92
Grade: 88

Looping Over Key-Value Pairs

To get both keys and values, use .items():

grades = {
    "Math": 90,
    "Science": 85,
    "English": 92,
    "History": 88
}

for subject, grade in grades.items():
    print(f"{subject}: {grade}")

Output:

Math: 90
Science: 85
English: 92
History: 88

This is the most common and useful pattern.

Practical Examples

Example 1: Calculate total and average

prices = {
    "laptop": 999.99,
    "mouse": 24.99,
    "keyboard": 79.99,
    "monitor": 299.99
}

total = 0
for item, price in prices.items():
    print(f"{item}: ${price:.2f}")
    total += price

print(f"\nTotal: ${total:.2f}")
average = total / len(prices)
print(f"Average price: ${average:.2f}")

Example 2: Find highest value

scores = {
    "Alice": 95,
    "Bob": 87,
    "Charlie": 92,
    "Diana": 98
}

highest_name = ""
highest_score = 0

for name, score in scores.items():
    if score > highest_score:
        highest_score = score
        highest_name = name

print(f"Highest score: {highest_name} with {highest_score}")

Example 3: Filter and display

inventory = {
    "sword": 1,
    "shield": 1,
    "potion": 15,
    "gold_coin": 250,
    "mana_potion": 8
}

print("Items with quantity > 5:")
for item, quantity in inventory.items():
    if quantity > 5:
        print(f"  {item}: {quantity}")

Example 4: Count and categorize

ages = {
    "Alice": 12,
    "Bob": 15,
    "Charlie": 11,
    "Diana": 14,
    "Eve": 16
}

teens = 0
for name, age in ages.items():
    if age >= 13:
        print(f"{name} is a teenager ({age})")
        teens += 1

print(f"\nTotal teenagers: {teens}")

Dictionary Helpers 🛠️

Python provides several useful functions and methods for working with dictionaries.

The `len()` Function

Get the number of key-value pairs:

student = {
    "name": "Jordan",
    "age": 14,
    "grade": "9th",
    "gpa": 3.8
}

count = len(student)
print(f"The dictionary has {count} entries")  # Shows: 4 entries

Getting All Keys or Values

Convert keys or values to lists:

grades = {"Math": 90, "Science": 85, "English": 92}

subjects = list(grades.keys())
scores = list(grades.values())

print(f"Subjects: {subjects}")
print(f"Scores: {scores}")

Copying Dictionaries

Create a copy with .copy():

original = {"name": "Alex", "score": 100}
backup = original.copy()

original["score"] = 200

print(f"Original: {original}")  # Shows: {'name': 'Alex', 'score': 200}
print(f"Backup: {backup}")      # Shows: {'name': 'Alex', 'score': 100}

Without .copy(), both variables would point to the same dictionary.

Merging Dictionaries

Combine two dictionaries:

defaults = {"theme": "dark", "language": "en", "notifications": True}
user_prefs = {"theme": "light", "font_size": 14}

# Python 3.9+: Use the | operator
settings = defaults | user_prefs
print(settings)
# Shows: {'theme': 'light', 'language': 'en', 'notifications': True, 'font_size': 14}

# Alternative: use .update()
all_settings = defaults.copy()
all_settings.update(user_prefs)

Checking for Keys

We already saw in, but here are more examples:

user = {"username": "alex123", "email": "alex@example.com"}

# Check if key exists
if "username" in user:
    print(f"Username: {user['username']}")

# Check if key doesn't exist
if "phone" not in user:
    print("No phone number on file")

Nested Dictionaries 🏗️

Dictionaries can contain other dictionaries, allowing you to create complex data structures.

Basic Nesting

student = {
    "name": "Jordan",
    "age": 14,
    "grades": {
        "Math": 90,
        "Science": 85,
        "English": 92
    },
    "contact": {
        "email": "jordan@example.com",
        "phone": "555-0123"
    }
}

# Access nested values
print(student["name"])                    # Shows: Jordan
print(student["grades"]["Math"])          # Shows: 90
print(student["contact"]["email"])        # Shows: jordan@example.com

Multiple Entries

# Multiple students
students = {
    "student1": {
        "name": "Alice",
        "grade": 90,
        "year": "10th"
    },
    "student2": {
        "name": "Bob",
        "grade": 85,
        "year": "9th"
    },
    "student3": {
        "name": "Charlie",
        "grade": 92,
        "year": "10th"
    }
}

# Loop through nested dictionaries
for student_id, info in students.items():
    print(f"{student_id}:")
    print(f"  Name: {info['name']}")
    print(f"  Grade: {info['grade']}")
    print(f"  Year: {info['year']}")
    print()

Quick Recap 🎯

You’ve mastered dictionaries in Python:

Create dictionaries with {} and key-value pairs
Access values using keys in square brackets or .get()
Add and update entries by assigning to keys
Remove entries with del, .pop(), or .clear()
Loop through keys, values, or items with .items()
Use helpers like len(), in, .keys(), and .values()
Nest dictionaries to create complex data structures

Dictionaries make organizing related data intuitive and efficient!

Hands-On Mini Project: Contact Book 📇

Let’s build a contact management system that uses dictionaries to store and organize contact information.

Your Mission

Create an interactive contact book where users can:

Add new contacts
View all contacts
Search for a contact
Update contact information
Delete a contact
Exit the program

Example Solution

print("=" * 60)
print("              CONTACT BOOK MANAGER")
print("=" * 60)
print()

# Initialize empty contact book
contacts = {}

# Main program loop
running = True

while running:
    # Display menu
    print("\nWhat would you like to do?")
    print("1. Add a contact")
    print("2. View all contacts")
    print("3. Search for a contact")
    print("4. Update a contact")
    print("5. Delete a contact")
    print("6. Exit")
    
    choice = input("\nEnter your choice (1-6): ")
    print()
    
    # Add a contact
    if choice == "1":
        print("=== Add New Contact ===")
        name = input("Enter name: ").strip()
        
        if name in contacts:
            print(f"❌ {name} already exists!")
        else:
            phone = input("Enter phone number: ").strip()
            email = input("Enter email: ").strip()
            
            contacts[name] = {
                "phone": phone,
                "email": email
            }
            
            print(f"✓ Added {name} to contacts!")
    
    # View all contacts
    elif choice == "2":
        if len(contacts) == 0:
            print("Your contact book is empty.")
        else:
            print("=" * 60)
            print("ALL CONTACTS:")
            print("=" * 60)
            
            for name, info in contacts.items():
                print(f"\n📇 {name}")
                print(f"   Phone: {info['phone']}")
                print(f"   Email: {info['email']}")
            
            print("\n" + "=" * 60)
            print(f"Total contacts: {len(contacts)}")
    
    # Search for a contact
    elif choice == "3":
        search_name = input("Enter name to search: ").strip()
        
        if search_name in contacts:
            info = contacts[search_name]
            print("\n" + "=" * 60)
            print(f"📇 {search_name}")
            print(f"   Phone: {info['phone']}")
            print(f"   Email: {info['email']}")
            print("=" * 60)
        else:
            print(f"❌ {search_name} not found in contacts.")
    
    # Update a contact
    elif choice == "4":
        update_name = input("Enter name to update: ").strip()
        
        if update_name not in contacts:
            print(f"❌ {update_name} not found in contacts.")
        else:
            print(f"\nCurrent information for {update_name}:")
            print(f"Phone: {contacts[update_name]['phone']}")
            print(f"Email: {contacts[update_name]['email']}")
            print()
            
            print("Enter new information (press Enter to keep current):")
            new_phone = input(f"Phone [{contacts[update_name]['phone']}]: ").strip()
            new_email = input(f"Email [{contacts[update_name]['email']}]: ").strip()
            
            # Update only if new value provided
            if new_phone:
                contacts[update_name]['phone'] = new_phone
            if new_email:
                contacts[update_name]['email'] = new_email
            
            print(f"✓ Updated {update_name}!")
    
    # Delete a contact
    elif choice == "5":
        delete_name = input("Enter name to delete: ").strip()
        
        if delete_name not in contacts:
            print(f"❌ {delete_name} not found in contacts.")
        else:
            confirm = input(f"Are you sure you want to delete {delete_name}? (yes/no): ")
            if confirm.lower() == "yes":
                contacts.pop(delete_name)
                print(f"✓ Deleted {delete_name} from contacts.")
            else:
                print("Cancelled.")
    
    # Exit
    elif choice == "6":
        print(f"You have {len(contacts)} contact(s) in your book.")
        print("\nThanks for using Contact Book Manager!")
        print("Goodbye! 👋")
        running = False
    
    # Invalid choice
    else:
        print("❌ Invalid choice. Please enter 1-6.")

print()
print("=" * 60)

Challenge Yourself

Enhance your contact book with these features:

Multiple phone numbers — Store home, work, and mobile numbers
Address information — Add street, city, state, zip
Categories — Tag contacts as family, friends, work, etc.
Birthday tracking — Store and display birthdays
Favorites — Mark certain contacts as favorites
Export contacts — Display all contacts in a formatted way to copy
Advanced search — Search by phone number or email, not just name
Sort contacts — Display contacts alphabetically
Notes field — Add custom notes to each contact
Statistics — Show how many contacts in each category

Tips for Building

Use .strip() on input to remove extra spaces
Check if a contact exists before trying to update or delete
Validate input (e.g., ensure email contains @)
Use nested dictionaries for complex contact information
Give clear feedback after each operation
Consider using .get() for safer access to nested values

What’s Next?

In the next chapter, you’ll learn about functions — how to create your own reusable blocks of code! You’ll discover how to organize your programs better and avoid repeating yourself.

Get ready to write cleaner, more organized code!

Excellent work! Dictionaries are incredibly powerful for organizing real-world data!

Chapter 8: Dictionaries: Key-Value Pairs 📖

Introduction to Dictionaries 🎯

Creating Dictionaries

More Examples

Why Use Dictionaries?

Displaying Dictionaries

Accessing Dictionary Values 🔍

Using Keys

Keys Are Case-Sensitive

Using .get() Method

Using Dictionary Values

Checking if a Key Exists

Adding and Updating Entries ✏️

Adding New Entries

Updating Existing Entries

Using .update() Method

Practical Example: Character Stats

Removing Entries 🗑️

Using del Statement

Using .pop() Method

Using .clear() Method

Looping Through Dictionaries 🔁

Looping Over Keys

Looping Over Values

Looping Over Key-Value Pairs

Practical Examples

Dictionary Helpers 🛠️

The len() Function

Getting All Keys or Values

Copying Dictionaries

Merging Dictionaries

Checking for Keys

Nested Dictionaries 🏗️

Basic Nesting

Multiple Entries

Quick Recap 🎯

Hands-On Mini Project: Contact Book 📇

Your Mission

Example Solution

Challenge Yourself

Tips for Building

What’s Next?

Using `.get()` Method

Using `.update()` Method

Using `del` Statement

Using `.pop()` Method

Using `.clear()` Method

The `len()` Function