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Python Dictionaries

Think of a dictionary in Python like a real-world dictionary where you look up definitions. Instead of looking up word definitions, you can store and look up any kind of information using a unique "key".

For example, just like you look up the meaning of "apple" in a dictionary, in Python you can store and look up a person's age using their name as the key.

What You'll Learn

✏️ Basics

  • • What dictionaries are and why they're useful
  • • How to create and use dictionaries
  • • Working with keys and values

🔧 Operations

  • • Adding and updating items
  • • Removing items
  • • Common dictionary methods

Understanding Dictionaries

A dictionary is a collection of key-value pairs. Each key must be unique, but values can be repeated. Here's a simple example:

Python
1# A simple dictionary storing ages
2ages = {
3    "John": 25,
4    "Alice": 22,
5    "Bob": 30
6}

In this example:

  • • The names ("John", "Alice", "Bob") are the keys
  • • The ages (25, 22, 30) are the values
  • • Each key-value pair is separated by a comma
  • • The entire dictionary is enclosed in curly braces

Creating Your First Dictionary

Let's start with the basics of creating dictionaries:

Python
1# Creating an empty dictionary
2my_dict = {}
3

4# Creating a dictionary with initial values
5student = {
6    "name": "Alice",
7    "age": 20,
8    "grade": "A"
9}
10

11# Print the dictionary
12print(student)  # Output: {'name': 'Alice', 'age': 20, 'grade': 'A'}

Important Notes:

  • • Keys must be unique
  • • Keys must be immutable (strings, numbers, or tuples)
  • • Values can be of any type (numbers, strings, lists, other dictionaries)

Accessing Dictionary Values

There are two main ways to access values in a dictionary:

Python
1# Create a dictionary
2student = {
3    "name": "Alice",
4    "age": 20,
5    "grade": "A"
6}
7

8# Method 1: Using square brackets
9name = student["name"]
10print(name)  # Output: Alice
11

12# Method 2: Using get() method (safer)
13age = student.get("age")
14print(age)  # Output: 20
15

16# Using get() with a default value
17score = student.get("score", 0)  # Returns 0 if "score" doesn't exist
18print(score)  # Output: 0

🚨 Common Mistake to Avoid

Using square brackets [] with a non-existent key will raise an error. It's safer to use the get() method, which returns None or a default value if the key doesn't exist.

Adding and Updating Values

You can easily add new items or update existing ones:

Python
1# Start with an empty dictionary
2student = {}
3

4# Adding new key-value pairs
5student["name"] = "Alice"
6student["age"] = 20
7

8# Update existing value
9student["age"] = 21
10

11print(student)  # Output: {'name': 'Alice', 'age': 21}
12

13# Add multiple items at once using update()
14student.update({
15    "grade": "A",
16    "city": "London"
17})
18

19print(student)  # Output: {'name': 'Alice', 'age': 21, 'grade': 'A', 'city': 'London'}

👋 Let's Practice!

Try these simple exercises to get comfortable with dictionaries:

Python
1# Exercise 1: Create a dictionary about your favorite book
2# Include: title, author, year, and rating
3

4# Exercise 2: Create a simple contact list
5contacts = {}
6# Add at least 3 contacts with their phone numbers
7# Update one contact's number
8# Print all contacts
9

10# Exercise 3: Create a mini library catalog
11# Store books with their status (available/borrowed)
12# Add functions to:
13#   - Add new books
14#   - Mark books as borrowed/returned
15#   - Check if a book is available

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