Why list comprehension is used in Python - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a list of squares from 1 to 5 using list comprehension.

Python

squares = [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 * instead of ** will multiply x by 2, not square it.

The ** operator is used to raise a number to a power. Here, x**2 means x squared.

2fill in blank

medium

Complete the code to create a list of even numbers from 0 to 10 using list comprehension.

Python

evens = [x for x in range(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 % will do floor division, not remainder.

The % operator gives the remainder. If x % 2 == 0, x is even.

3fill in blank

hard

Fix the error in the list comprehension to create a list of uppercase words.

Python

words = ['apple', 'banana', 'cherry']
upper_words = [word[1] for word in words]
print(upper_words)

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

A.capitalize()

B.lower()

C.upper()

D.title()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .lower() will make all letters lowercase, not uppercase.

The .upper() method converts a string to uppercase letters.

4fill in blank

hard

Fill both blanks to create a list of squares for numbers greater than 3.

Python

result = [x[1]2 for x in range(1, 7) if x [2] 3]
print(result)

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

A**

B>

C<

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using * instead of ** will multiply by 2, not square.

Using < instead of > will select numbers less than 3.

Use ** to square numbers and > to filter numbers greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary of word lengths for words longer than 4 letters.

Python

word_lengths = [1]: [1] for [2] in words if len([3]) > 4}

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

Aword

Blen(word)

Cwords

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for keys and loop variable causes errors.

Using 'words' as loop variable is incorrect; it is the list.

The dictionary keys are words (word), values are their lengths (len(word)), and the loop variable is word.

Practice

1. Why do programmers use list comprehension in Python?

easy

A. To make code run slower

B. To write longer code for better understanding

C. To avoid using loops completely

D. To create lists in a shorter and clearer way

Solution

1. Step 1: Understand the purpose of list comprehension

   List comprehension is designed to make list creation concise and readable.

2. Step 2: Compare options with this purpose

   Only To create lists in a shorter and clearer way says it helps create lists shorter and clearer, which matches the purpose.

3. Final Answer:

   To create lists in a shorter and clearer way -> Option D

4. Quick Check:

   List comprehension = shorter, clearer list creation [OK]

Hint: List comprehension makes list creation short and clear [OK]

Common Mistakes:

• Thinking it avoids loops completely
• Believing it makes code slower
• Assuming it makes code longer

2. Which of the following is the correct syntax for a simple list comprehension that squares numbers from 1 to 3?

easy

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

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

C. [x**2 in range(1, 4)]

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

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is: [expression for variable in iterable]

2. Step 2: Check each option

   [x**2 for x in range(1, 4)] matches the correct syntax; others have syntax errors or missing brackets.

3. Final Answer:

   [x**2 for x in range(1, 4)] -> Option B

4. Quick Check:

   Correct syntax = [x**2 for x in range(1, 4)] [OK]

Hint: Remember: [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after the expression
• Missing brackets around comprehension
• Using colon instead of brackets

3. What is the output of this code?

nums = [1, 2, 3, 4]
squares = [n*n for n in nums if n % 2 == 0]
print(squares)

medium

A. [4, 16]

B. [2, 4]

C. [1, 4, 9, 16]

D. [1, 9]

Solution

1. Step 1: Identify the list and condition

   The list is nums = [1, 2, 3, 4]. The comprehension squares numbers only if they are even (n % 2 == 0).

2. Step 2: Calculate squares of even numbers

   Even numbers are 2 and 4. Their squares are 4 and 16.

3. Final Answer:

   [4, 16] -> Option A

4. Quick Check:

   Squares of even nums = [4, 16] [OK]

Hint: Filter first, then apply expression in comprehension [OK]

Common Mistakes:

• Including odd numbers by mistake
• Returning original numbers instead of squares
• Confusing condition placement

4. Find the error in this list comprehension:

result = [x*2 for x in range(5) if x > 2 else x]

medium

A. Using * instead of ** for power

B. Missing brackets around the expression

C. The else clause is not allowed in this position

D. range(5) should be range(1,5)

Solution

1. Step 1: Understand list comprehension with condition

   When using if-else inside list comprehension, it must be part of the expression, not after the for loop.

2. Step 2: Identify the syntax error

   The else clause after the if in the comprehension is invalid syntax here; it should be inside the expression part.

3. Final Answer:

   The else clause is not allowed in this position -> Option C

4. Quick Check:

   if-else must be inside expression, not after for [OK]

Hint: if-else goes inside expression, not after for [OK]

Common Mistakes:

• Placing else after the for loop
• Confusing * with ** for multiplication
• Changing range unnecessarily

5. You have a list of words: words = ['apple', '', 'banana', ' ', 'cherry']. Using list comprehension, how can you create a new list that contains only non-empty and non-blank words?

hard

A. [word for word in words if word.strip()]

B. [word for word in words if word]

C. [word for word in words if word != '']

D. [word for word in words if len(word) > 1]

Solution

1. Step 1: Understand the filtering requirement

   We want to exclude empty strings and strings with only spaces (blank).

2. Step 2: Analyze each option's filter

   [word for word in words if word] excludes empty strings but keeps strings with spaces. [word for word in words if word != ''] excludes empty strings only. [word for word in words if word.strip()] uses strip() to remove spaces and checks if anything remains, filtering out blanks. [word for word in words if len(word) > 1] excludes words of length 1 or less, which wrongly excludes single-letter words.

3. Final Answer:

   [word for word in words if word.strip()] -> Option A

4. Quick Check:

   Use strip() to remove blanks before filtering [OK]

Hint: Use word.strip() to filter out blanks and empty strings [OK]

Common Mistakes:

• Only checking if word is non-empty, missing blanks
• Filtering by length incorrectly
• Not using strip() to remove spaces