Accessing values using keys in Python - Mini Project: Build & Apply

Accessing values using keys

📖 Scenario: You are managing a small library system. You have a dictionary that stores book titles as keys and their authors as values.

🎯 Goal: You will create a dictionary of books and authors, then access the author of a specific book using its title as the key.

📋 What You'll Learn

Create a dictionary called library with exact book titles and authors

Create a variable called book_title with the exact book title to look up

Access the author of the book using the book_title key from the library dictionary

Print the author name

💡 Why This Matters

🌍 Real World

Dictionaries are used to store and look up data quickly, like finding a book's author by its title in a library system.

💼 Career

Understanding how to access values using keys is essential for data handling in many programming jobs, including web development, data analysis, and software engineering.

1

Create the library dictionary

Create a dictionary called library with these exact entries: 'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'

Python

# Create the library dictionary with book titles and authors
# Your code here

Hint

Use curly braces {} to create a dictionary with keys and values separated by colons.

2

Set the book title to look up

Create a variable called book_title and set it to the string '1984'

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
# Create the variable book_title with the value '1984'
# Your code here

Hint

Use an equals sign = to assign the string '1984' to the variable book_title.

3

Access the author using the book title key

Create a variable called author and set it to the value from the library dictionary using the key stored in book_title

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
book_title = '1984'
# Access the author from library using book_title as the key
# Your code here

Hint

Use square brackets [] with the dictionary name and the key variable inside to get the value.

4

Print the author

Write a print statement to display the value stored in the variable author

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
book_title = '1984'
author = library[book_title]
# Print the author
# Your code here

Hint

Use print(author) to show the author's name on the screen.

Practice

1. What is the correct way to access the value associated with the key 'name' in the dictionary person = {'name': 'Alice', 'age': 30}?

easy

A. person.get('age')

B. person.name

C. person['name']

D. person['Alice']

Solution

1. Step 1: Identify the dictionary and key

   The dictionary is person and the key to access is 'name'.

2. Step 2: Use correct syntax to access value by key

   In Python, dictionary values are accessed using square brackets and the key as a string: person['name'].

3. Final Answer:

   person['name'] -> Option C

4. Quick Check:

   Access value by key = person['name'] [OK]

Hint: Use square brackets with the key string to get value [OK]

Common Mistakes:

• Using dot notation like person.name (not valid for dict)
• Using a value instead of key inside brackets
• Accessing a different key than asked

2. Which of the following is the correct syntax to safely get the value for key 'city' from dictionary data without causing an error if the key does not exist?

easy

A. data['city']

B. data.get('city')

C. data.city

D. data['City']

Solution

1. Step 1: Understand safe access in dictionaries

   Using data['city'] causes an error if the key is missing. Using data.get('city') returns None instead.

2. Step 2: Identify correct method for safe access

   The get() method is designed to safely access keys without errors.

3. Final Answer:

   data.get('city') -> Option B

4. Quick Check:

   Safe key access = data.get('city') [OK]

Hint: Use get() to avoid errors if key missing [OK]

Common Mistakes:

• Using square brackets which raise KeyError if key missing
• Using dot notation which is invalid for dict
• Using wrong key case causing KeyError

3. What will be the output of this code?

info = {'a': 1, 'b': 2, 'c': 3}
print(info['b'])

medium

A. 2

B. 1

C. 3

D. KeyError

Solution

1. Step 1: Identify the dictionary and key accessed

   The dictionary info has key 'b' with value 2.

2. Step 2: Access the value using the key

   The code prints info['b'], which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   info['b'] = 2 [OK]

Hint: Look up the key's value directly in the dictionary [OK]

Common Mistakes:

• Confusing key with value
• Expecting KeyError when key exists
• Mixing up keys and values

4. The following code causes an error. What is the problem?

data = {'x': 10, 'y': 20}
print(data['z'])

medium

A. TypeError because keys must be integers

B. SyntaxError due to wrong brackets

C. No error, prints 0

D. KeyError because 'z' is not in the dictionary

Solution

1. Step 1: Check if key exists in dictionary

   The key 'z' is not present in data.

2. Step 2: Understand error caused by missing key

   Accessing a missing key with square brackets raises a KeyError.

3. Final Answer:

   KeyError because 'z' is not in the dictionary -> Option D

4. Quick Check:

   Missing key access = KeyError [OK]

Hint: Check if key exists before accessing or use get() [OK]

Common Mistakes:

• Assuming missing keys return 0 or None
• Confusing syntax error with runtime error
• Using wrong bracket types

5. Given the dictionary grades = {'Alice': 85, 'Bob': 92, 'Charlie': 78}, which code snippet correctly prints Bob's grade or 'Not found' if Bob is not in the dictionary?

hard

A. print(grades.get('Bob', 'Not found'))

B. print(grades['Bob'] or 'Not found')

C. print(grades['bob'] or 'Not found')

D. print(grades.get('Bob'))

Solution

1. Step 1: Understand the goal

   We want to print Bob's grade if present, otherwise print 'Not found'.

2. Step 2: Analyze each option

   print(grades['Bob'] or 'Not found') will raise KeyError if key missing because access happens before 'or'. print(grades.get('Bob', 'Not found')) uses get() with default value, which is concise and safe. print(grades['bob'] or 'Not found') uses wrong key case and will fail. print(grades.get('Bob')) prints None if key missing, not 'Not found'.

3. Final Answer:

   print(grades.get('Bob', 'Not found')) -> Option A

4. Quick Check:

   Use get() with default for safe access [OK]

Hint: Use get(key, default) to handle missing keys easily [OK]

Common Mistakes:

• Using wrong key case causing missing key
• Not providing default value in get()
• Assuming or operator works for missing keys