beginner

Python Dictionaries

Learn to store and access data using key-value pairs with Python dictionaries.

Dictionaries are one of Python's most powerful built-in data structures. Unlike lists which use numeric indexes, dictionaries let you access values using descriptive keys. This makes your code more readable and your data easier to manage. From configuration files to JSON APIs, dictionaries are used everywhere in real-world Python applications.

📚 Concepts & Theory

What is a Dictionary?

A dictionary is a collection of key-value pairs. Each key is unique and maps to a specific value.

student = {
    "name": "Alice",
    "age": 20,
    "grade": "A"
}

Accessing Values:

print(student["name"])  # Alice
print(student.get("age"))  # 20

Adding and Modifying:

student["email"] = "[email protected]"  # Add new
student["age"] = 21  # Modify existing

Common Dictionary Methods:

keys() - Get all keys
values() - Get all values
items() - Get key-value pairs
get(key, default) - Safe access with default
pop(key) - Remove and return value

🎯 Your Challenge

Create a dictionary called `book` with three keys: "title" set to "Python Basics", "author" set to "John Smith", and "pages" set to 350. Then add a new key "year" with the value 2024.

📝 Starter Code

Python

# Create the book dictionary
book = {
    # Add your key-value pairs here
}

# Add the year key

Dictionaries use curly braces {}
Keys and values are separated by colons :
String keys need quotes
Use bracket notation to add new keys

Solution

Python

book = {
    "title": "Python Basics",
    "author": "John Smith",
    "pages": 350
}

book["year"] = 2024

Explanation

We create a dictionary using curly braces {} with key-value pairs separated by colons. Keys are strings in quotes, and values can be any data type. To add a new key after creation, we use bracket notation with assignment.

⚠️ Common Mistakes to Avoid

Using square brackets instead of curly braces for dictionary creation
Forgetting quotes around string keys
Using = instead of : inside the dictionary
Forgetting commas between key-value pairs

❓ Frequently Asked Questions

Yes, dictionary keys can be any immutable type including numbers, strings, and tuples. However, strings are most commonly used for readability.

Using bracket notation raises a KeyError. Use the get() method with a default value to avoid this error.