Basic dictionary comprehension in Python - Mini Project: Build & Apply

Basic dictionary comprehension

📖 Scenario: You work in a small bookstore. You have a list of books with their prices. You want to create a new dictionary that shows the prices after a discount.

🎯 Goal: Create a dictionary using dictionary comprehension to apply a discount to each book price.

📋 What You'll Learn

Create a dictionary called books with given book titles and prices.

Create a variable called discount to hold the discount rate.

Use dictionary comprehension to create a new dictionary called discounted_books with discounted prices.

Print the discounted_books dictionary.

💡 Why This Matters

🌍 Real World

Applying discounts to product prices is common in stores and online shops.

💼 Career

Understanding dictionary comprehension helps in data processing and automation tasks in many programming jobs.

1

Create the initial dictionary of books

Create a dictionary called books with these exact entries: 'Python Basics': 40, 'Data Science': 50, 'Machine Learning': 60

Python

# Create the books dictionary with exact titles and prices
# Your code here

Hint

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

2

Set the discount rate

Create a variable called discount and set it to 0.1 to represent a 10% discount

Python

books = {'Python Basics': 40, 'Data Science': 50, 'Machine Learning': 60}
# Create a discount variable with value 0.1
# Your code here

Hint

Just assign the number 0.1 to the variable discount.

3

Create discounted prices with dictionary comprehension

Use dictionary comprehension to create a new dictionary called discounted_books where each price is reduced by the discount. Use for title, price in books.items() in the comprehension

Python

books = {'Python Basics': 40, 'Data Science': 50, 'Machine Learning': 60}
discount = 0.1
# Create discounted_books using dictionary comprehension
# Your code here

Hint

Use the format {key: value_expression for key, value in dictionary.items()}.

4

Print the discounted books dictionary

Write print(discounted_books) to display the dictionary with discounted prices

Python

books = {'Python Basics': 40, 'Data Science': 50, 'Machine Learning': 60}
discount = 0.1
discounted_books = {title: price * (1 - discount) for title, price in books.items()}
# Print the discounted_books dictionary
# Your code here

Hint

Use the print function to show the dictionary.

Practice

1. What does the following dictionary comprehension do?
{x: x*x for x in range(3)}

easy

A. Creates a list of squares from 0 to 2

B. Creates a set of squares from 0 to 2

C. Creates a dictionary with numbers 0 to 2 as keys and their squares as values

D. Creates a dictionary with keys as squares and values as numbers

Solution

1. Step 1: Understand the loop and key-value pairs

   The comprehension loops over numbers 0, 1, 2 and uses each number as the key.

2. Step 2: Calculate the value for each key

   Each value is the square of the key (x*x), so values are 0, 1, 4 respectively.

3. Final Answer:

   Creates a dictionary with numbers 0 to 2 as keys and their squares as values -> Option C

4. Quick Check:

   Dictionary comprehension = key: x, value: x*x [OK]

Hint: Look for key:value pairs inside braces with a for loop [OK]

Common Mistakes:

• Confusing dictionary with list or set comprehension
• Mixing keys and values in comprehension
• Thinking it creates a list instead of a dictionary

2. Which of the following is the correct syntax for a dictionary comprehension that creates keys from 1 to 3 and values as their double?

easy

A. [x: 2*x for x in range(1, 4)]

B. {x: 2*x for x in range(1, 4)}

C. {x => 2*x for x in range(1, 4)}

D. (x: 2*x for x in range(1, 4))

Solution

1. Step 1: Identify correct dictionary comprehension syntax

   Dictionary comprehensions use curly braces with key:value pairs and a for loop inside.

2. Step 2: Check each option

   {x: 2*x for x in range(1, 4)} uses curly braces and colon correctly. Options B, C, D use wrong brackets or arrow syntax.

3. Final Answer:

   {x: 2*x for x in range(1, 4)} -> Option B

4. Quick Check:

   Curly braces + key:value + for loop = correct syntax [OK]

Hint: Remember dictionary comprehension uses curly braces and colon [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using arrow (=>) instead of colon for key-value
• Using parentheses which create generator expressions

3. What is the output of this code?

nums = [1, 2, 3]
squares = {n: n**2 for n in nums if n % 2 != 0}
print(squares)

medium

A. {1: 1, 3: 9}

B. {2: 4}

C. {1: 1, 2: 4, 3: 9}

D. {}

Solution

1. Step 1: Understand the filtering condition

   The comprehension includes only numbers where n % 2 != 0, meaning odd numbers only (1 and 3).

2. Step 2: Calculate squares for filtered numbers

   Squares are 1*1=1 and 3*3=9, so dictionary is {1:1, 3:9}.

3. Final Answer:

   {1: 1, 3: 9} -> Option A

4. Quick Check:

   Filter odd numbers and square them = {1:1, 3:9} [OK]

Hint: Check the if condition filters keys before squaring [OK]

Common Mistakes:

• Including even numbers by mistake
• Confusing list comprehension output with dictionary
• Ignoring the if condition in comprehension

4. Find the error in this dictionary comprehension:

result = {x, x*2 for x in range(3)}

medium

A. Missing colon between key and value

B. Using parentheses instead of curly braces

C. Range function is incorrect

D. No error, code is correct

Solution

1. Step 1: Check syntax for key-value pairs

   Dictionary comprehension requires a colon ':' between key and value, but here a comma ',' is used.

2. Step 2: Confirm other parts are correct

   Curly braces and range(3) are correct, so the error is the missing colon.

3. Final Answer:

   Missing colon between key and value -> Option A

4. Quick Check:

   Key:value pairs need colon, not comma [OK]

Hint: Use colon ':' to separate key and value in dict comprehension [OK]

Common Mistakes:

• Using comma instead of colon between key and value
• Confusing set comprehension with dictionary comprehension
• Using wrong brackets for comprehension

5. Given a list of words, create a dictionary comprehension that maps each word to its length but only include words longer than 3 letters.
Which code correctly does this?

words = ['cat', 'house', 'dog', 'elephant']

hard

A. {word: len(word) for word in words where len(word) > 3}

B. {word: len(word) for word in words if word > 3}

C. {word: len(word) if len(word) > 3 for word in words}

D. {word: len(word) for word in words if len(word) > 3}

Solution

1. Step 1: Understand filtering condition syntax

   The if condition must come after the for loop to filter words by length correctly.

2. Step 2: Check each option's syntax

   {word: len(word) for word in words if len(word) > 3} correctly places 'if len(word) > 3' after the for loop. Others have syntax errors or wrong condition placement.

3. Final Answer:

   {word: len(word) for word in words if len(word) > 3} -> Option D

4. Quick Check:

   Filter after for loop with if condition [OK]

Hint: Put if condition after for loop in dictionary comprehension [OK]

Common Mistakes:

• Placing if condition before for loop
• Using invalid syntax like 'where' instead of 'if'
• Comparing word string directly to number