Why dictionary comprehension is used in Python - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a dictionary with numbers as keys and their squares as values.

Python

squares = {x: x[1]2 for x in range(1, 6)}
print(squares)

Drag options to blanks, or click blank then click option'

A+

B**

C*

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication operator instead of exponentiation.

The ** operator is used to raise a number to a power. Here, it squares the number.

2fill in blank

medium

Complete the code to create a dictionary of words and their lengths from a list.

Python

words = ['apple', 'banana', 'cherry']
lengths = {word: [1] for word in words}
print(lengths)

Drag options to blanks, or click blank then click option'

Aword

Blength(word)

Cword.length

Dlen(word)

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect syntax like word.length which is not valid in Python.

The len() function returns the length of a string in Python.

3fill in blank

hard

Fix the error in the dictionary comprehension to include only even numbers as keys.

Python

evens = {x: x**2 for x in range(1, 11) if x [1] 2 == 0}
print(evens)

Drag options to blanks, or click blank then click option'

A%

B!=

C//

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using != instead of % for checking even numbers.

The modulo operator % gives the remainder. If remainder is 0 when divided by 2, the number is even.

4fill in blank

hard

Fill both blanks to create a dictionary of numbers and their cubes only if the cube is greater than 10.

Python

cubes = {x: x[1]3 for x in range(1, 6) if x[2]3 > 10}
print(cubes)

Drag options to blanks, or click blank then click option'

A**

B*

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of exponentiation.

Using wrong comparison operator.

Use ** to cube the number and > to check if the cube is greater than 10.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values only if the length of the word is greater than 4.

Python

words = ['dog', 'elephant', 'cat', 'giraffe']
result = [1]: [2] for word in words if len(word) [3] 4
print(result)

Drag options to blanks, or click blank then click option'

Aword.upper()

Bword

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator.

Not converting keys to uppercase.

Use word.upper() to make keys uppercase, keep values as word, and filter words with length greater than 4 using >.

Practice

1. Why do programmers use dictionary comprehension in Python?

easy

A. To avoid using loops completely

B. To create dictionaries quickly and in a single line

C. To write longer code for clarity

D. To create lists instead of dictionaries

Solution

1. Step 1: Understand dictionary comprehension purpose

   Dictionary comprehension is designed to create dictionaries quickly and concisely in one line.

2. Step 2: Compare options with this purpose

   To create dictionaries quickly and in a single line matches this purpose, while others are incorrect or unrelated.

3. Final Answer:

   To create dictionaries quickly and in a single line -> Option B

4. Quick Check:

   Dictionary comprehension = fast dictionary creation [OK]

Hint: Dictionary comprehension makes dicts fast and short [OK]

Common Mistakes:

• Thinking it creates lists
• Believing it avoids loops entirely
• Assuming it makes code longer

2. Which of the following is the correct syntax for a dictionary comprehension?

easy

A. {key: value for key, value in iterable}

B. [key: value for key, value in iterable]

C. (key: value for key, value in iterable)

D. {key, value for key, value in iterable}

Solution

1. Step 1: Recall dictionary comprehension syntax

   Dictionary comprehension uses curly braces with key:value pairs and a for loop inside.

2. Step 2: Match syntax to options

   {key: value for key, value in iterable} uses curly braces and correct key:value format; others use wrong brackets or separators.

3. Final Answer:

   {key: value for key, value in iterable} -> Option A

4. Quick Check:

   Dict comprehension syntax = curly braces with key:value [OK]

Hint: Use curly braces and colon for dict comprehension [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using parentheses which create generators
• Separating key and value with commas

3. What is the output of this code?

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

medium

A. {1: 1}

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

C. {2: 4, 3: 9}

D. {}

Solution

1. Step 1: Understand the dictionary comprehension with condition

   The comprehension includes only numbers greater than 1, so 2 and 3 are included.

2. Step 2: Calculate squares for included numbers

   2 squared is 4, 3 squared is 9, so the dictionary is {2: 4, 3: 9}.

3. Final Answer:

   {2: 4, 3: 9} -> Option C

4. Quick Check:

   Filter n > 1, squares = {2:4, 3:9} [OK]

Hint: Filter condition removes keys not matching [OK]

Common Mistakes:

• Including all numbers ignoring the condition
• Confusing keys and values
• Expecting an empty dictionary

4. Find the error in this dictionary comprehension:

data = [1, 2, 3]
result = {x, x*2 for x in data}

medium

A. Using comma instead of colon between key and value

B. Missing parentheses around the comprehension

C. Using square brackets instead of curly braces

D. No error, code is correct

Solution

1. Step 1: Check key-value separator in comprehension

   Dictionary comprehension requires colon ':' between key and value, not comma.

2. Step 2: Identify the error in given code

   The code uses comma, which is invalid syntax for dict comprehension.

3. Final Answer:

   Using comma instead of colon between key and value -> Option A

4. Quick Check:

   Dict comprehension needs ':' not ',' [OK]

Hint: Use colon ':' between key and value [OK]

Common Mistakes:

• Using comma instead of colon
• Confusing list/set comprehension syntax
• Assuming code runs without error

5. You have a list of words: words = ['apple', 'banana', '', 'cherry', None]. How can you use dictionary comprehension to create a dictionary with words as keys and their lengths as values, but only include non-empty and non-None words?

hard

A. {w: len(w) for w in words}

B. {w: len(w) for w in words if w != '' or w is not None}

C. {w: len(w) for w in words if w == '' and w is None}

D. {w: len(w) for w in words if w}

Solution

1. Step 1: Understand filtering condition for valid words

   We want to exclude empty strings and None, which are falsy values in Python.

2. Step 2: Use condition that keeps only truthy words

   Using if w filters out empty strings and None automatically.

3. Final Answer:

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

4. Quick Check:

   Filter with if w excludes empty and None [OK]

Hint: Use if w to filter out empty and None [OK]

Common Mistakes:

• Using incorrect or redundant conditions
• Including empty strings or None by mistake
• Not filtering at all