Python Basics · Lesson 9

Dictionaries

Learn how Python dictionaries store information using key-value pairs and how to access, add, update, remove and process data.

What you will learn

• Key-value pairs
• Creating dictionaries
• Accessing values
• Adding data
• Updating values
• get()
• keys()
• values()
• items()
• pop()
• Membership testing
• Nested dictionaries

Part 1

What is a dictionary?

A dictionary stores data as key-value pairs. Instead of accessing information using numeric positions like a list, you normally access dictionary values using their keys.

student = {
    "name": "Aman",
    "age": 21,
    "course": "Python"
}

print(student)

CHECK YOUR UNDERSTANDING

What type of structure is a Python dictionary?

Fill in the Blank

Complete the empty dictionary: student = _____

Keys and values

Each dictionary entry contains a key and a value. The key is used to identify the associated value.

student = {
    "name": "Aman",
    "age": 21
}

CHECK YOUR UNDERSTANDING

In {"name": "Aman"}, what is the key?

CHECK YOUR UNDERSTANDING

In {"age": 21}, what is the value?

Accessing dictionary values

You can access a value by placing its key inside square brackets.

student = {
    "name": "Aman",
    "age": 21,
    "course": "Python"
}

print(student["name"])

CHECK YOUR UNDERSTANDING

What does student["name"] return?

Fill in the Blank

Access the course: student["_____"]

Important

Updating a dictionary value

Assign a new value to an existing key to update the dictionary.

student = {
    "name": "Aman",
    "age": 21
}

student["age"] = 22

print(student)

CHECK YOUR UNDERSTANDING

What is the new value of student['age']?

Adding a new key-value pair

Assigning a value to a key that does not already exist adds a new entry.

student = {
    "name": "Aman",
    "age": 21
}

student["city"] = "Dehradun"

print(student)

CHECK YOUR UNDERSTANDING

What happens when student['city'] is assigned?

Fill in the Blank

Add an email: student["_____"] = "aman@example.com"

get()

The get() method retrieves a value using a key. It can also provide a default value when the key does not exist.

student = {
    "name": "Aman",
    "age": 21
}

print(student.get("name"))

CHECK YOUR UNDERSTANDING

What does student.get("name") return?

print(student.get("city", "Unknown"))

CHECK YOUR UNDERSTANDING

If city does not exist, what will student.get("city", "Unknown") return?

Fill in the Blank

Get a value safely: student.get("city", "Unknown")

keys()

The keys() method provides the dictionary keys.

student = {
    "name": "Aman",
    "age": 21,
    "course": "Python"
}

print(student.keys())

CHECK YOUR UNDERSTANDING

What does keys() provide?

Fill in the Blank

Complete: student._____() to get the keys

values()

The values() method provides the values stored in a dictionary.

student = {
    "name": "Aman",
    "age": 21,
    "course": "Python"
}

print(student.values())

CHECK YOUR UNDERSTANDING

What does values() provide?

Fill in the Blank

Complete: student._____() to get the values

items()

The items() method provides dictionary entries as key-value pairs.

student = {
    "name": "Aman",
    "age": 21
}

print(student.items())

CHECK YOUR UNDERSTANDING

What does items() provide?

Fill in the Blank

Complete: student._____() to access key-value pairs

pop()

Dictionary pop() removes an entry using its key and returns its value.

student = {
    "name": "Aman",
    "age": 21,
    "course": "Python"
}

removed = student.pop("age")

print(removed)
print(student)

CHECK YOUR UNDERSTANDING

What value is stored in removed?

CHECK YOUR UNDERSTANDING

Which key is removed by student.pop('age')?

Fill in the Blank

Remove a key: student.pop("age")

Checking whether a key exists

The in operator can check whether a key exists in a dictionary.

student = {
    "name": "Aman",
    "age": 21
}

print("name" in student)

CHECK YOUR UNDERSTANDING

What does "name" in student return?

CHECK YOUR UNDERSTANDING

What does "city" in student return if city is not a key?

Predict the output

Think before running the code

product = {
    "name": "Laptop",
    "price": 55000
}

product["price"] = 60000

print(product["price"])

CHECK YOUR UNDERSTANDING

What will the program print?

Next level

Nested dictionaries

A dictionary can contain another dictionary as a value. This is useful for representing structured data.

student = {
    "name": "Aman",
    "details": {
        "age": 21,
        "city": "Dehradun"
    }
}

print(student["details"]["city"])

CHECK YOUR UNDERSTANDING

What will student["details"]["city"] return?

Fill in the Blank

Access the nested age value: student["details"]["_____"]

Debug the dictionary

student = {
    "name": "Aman",
    "age": 21
}

print(student["city"])

CHECK YOUR UNDERSTANDING

Why can this code cause a KeyError?

print(student.get("city", "Not available"))

Data Analytics Connection

Dictionaries and structured data

Dictionaries are useful for representing records where each field has a meaningful name.

sale = {
    "product": "Laptop",
    "quantity": 3,
    "price": 55000
}

revenue = sale["quantity"] * sale["price"]

print(revenue)

CHECK YOUR UNDERSTANDING

What is the revenue in this example?

Practice

Complete the dictionary operations

Fill in the Blank

Get a value safely: student.get("city", "Unknown")

Fill in the Blank

Get all keys: student._____()

Fill in the Blank

Get all values: student._____()

Fill in the Blank

Get key-value pairs: student._____()

Fill in the Blank

Remove a key: student.pop("age")

Mini Challenge

Build a student record

Create a dictionary containing a student's name, course and city. Add an age, update the city and then print the student's details.

student = {
    "name": "Priya",
    "course": "Data Analytics",
    "city": "Dehradun"
}

student["age"] = 22
student["city"] = "Delhi"

print(student)

Experiment by adding your own fields.

Final Challenge

Build a sales record

Create a dictionary containing a product, price and quantity. Calculate revenue from the dictionary values. Then add a category field and display all the keys.

sale = {
    "product": "Laptop",
    "price": 55000,
    "quantity": 2
}

revenue = sale["price"] * sale["quantity"]

sale["category"] = "Electronics"

print(revenue)
print(sale.keys())

Expected revenue: ₹110000

Lesson recap

✓ Dictionaries store key-value pairs.
✓ Keys are used to access values.
✓ New keys can be added by assignment.
✓ Existing values can be updated.
✓ get() safely retrieves values.
✓ keys() provides dictionary keys.
✓ values() provides dictionary values.
✓ items() provides key-value pairs.
✓ pop() removes an entry using its key.
✓ in can check whether a key exists.
✓ Dictionaries can contain nested dictionaries.
✓ Dictionaries are useful for structured records.