Basic list comprehension syntax in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is list comprehension in Python?

List comprehension is a concise way to create lists by writing a single expression inside square brackets, often replacing loops and append calls.

beginner

Write the basic syntax of a list comprehension.

The basic syntax is:
[expression for item in iterable]
where expression is the value to add, item is each element from iterable.

beginner

How does list comprehension compare to a for-loop for creating a list?

List comprehension is shorter and often easier to read. It combines the loop and list creation in one line, unlike a for-loop which needs multiple lines and an append call.

beginner

Example: What does this list comprehension do?
[x * 2 for x in range(3)]

It creates a list by multiplying each number from 0 to 2 by 2. The result is [0, 2, 4].

beginner

Can list comprehensions include conditions?

Yes, but that is a more advanced topic. The basic syntax does not include conditions. Conditions are added with an if clause inside the comprehension.

What is the output of this code?
[x + 1 for x in [1, 2, 3]]

A [2, 3, 4]

B [1, 2, 3]

C [0, 1, 2]

D [1, 3, 5]

Each element is increased by 1, so 1+1=2, 2+1=3, 3+1=4.

Which part of a list comprehension is the 'iterable'?

A The expression before 'for'

B The variable after 'for'

C The brackets []

D The sequence after 'in'

The iterable is the sequence or collection after the 'in' keyword.

What does this list comprehension do?
[x for x in range(5)]

A Creates a list of 5 zeros

B Creates a list of numbers 1 to 5

C Creates a list of numbers 0 to 4

D Creates an empty list

range(5) generates numbers 0 to 4, and the comprehension collects them as is.

Which of these is a valid list comprehension?

A for x in [1, 2, 3]: x * 2

B [x * 2 for x in [1, 2, 3]]

C [x * 2 in [1, 2, 3]]

D x * 2 for x in [1, 2, 3]

Only option B follows the correct list comprehension syntax.

What type of value does a list comprehension produce?

A A list

B A string

C An integer

D A dictionary

List comprehensions always produce a list.

Explain the basic syntax of a list comprehension and how it works.

Think about how you write a for-loop and how list comprehension shortens it.

Give an example of a simple list comprehension that doubles numbers from 0 to 4.

Use range and multiply each item by 2 inside the comprehension.

Practice

1. What does the following list comprehension do?
[x * 2 for x in range(3)]

easy

A. Creates a list of numbers from 1 to 6

B. Creates a list of numbers from 0 to 3

C. Creates a list of numbers doubled from 0 to 2

D. Creates a list of numbers squared from 0 to 2

Solution

1. Step 1: Understand the range and loop variable

   The range(3) generates numbers 0, 1, 2. The variable x takes these values one by one.

2. Step 2: Apply the expression to each value

   Each x is multiplied by 2, so the list becomes [0*2, 1*2, 2*2] = [0, 2, 4].

3. Final Answer:

   Creates a list of numbers doubled from 0 to 2 -> Option C

4. Quick Check:

   List comprehension doubles each number in range(3) [OK]

Hint: Remember: for x in range(3) gives 0,1,2 only [OK]

Common Mistakes:

• Thinking range(3) includes 3
• Confusing doubling with squaring
• Ignoring the for loop variable

2. Which of the following is the correct syntax for a list comprehension that creates a list of squares of numbers 0 to 4?

easy

A. [x**2 x in range(5)]

B. [for x in range(5) x**2]

C. [x^2 in range(5)]

D. [x**2 for x in range(5)]

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is [expression for variable in iterable]. Here, expression is x**2 and iterable is range(5).

2. Step 2: Check each option

   [x**2 for x in range(5)] matches the correct syntax. Options A, B, and C have syntax errors or wrong order.

3. Final Answer:

   [x**2 for x in range(5)] -> Option D

4. Quick Check:

   Correct syntax is [expr for var in iterable] [OK]

Hint: Remember: list comprehension = [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after expression
• Using ^ instead of ** for power
• Missing 'in' keyword

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. [1, 4, 9, 16]

C. [2, 4]

D. [1, 9]

Solution

1. Step 1: Understand the list and condition

   The list nums is [1, 2, 3, 4]. The condition 'if n % 2 == 0' selects even numbers: 2 and 4.

2. Step 2: Calculate squares of selected numbers

   Squares of 2 and 4 are 4 and 16, so the list squares is [4, 16].

3. Final Answer:

   [4, 16] -> Option A

4. Quick Check:

   Filter even numbers, then square them [OK]

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

Common Mistakes:

• Including odd numbers by mistake
• Confusing values with their squares
• Ignoring the if condition

4. Find the error in this list comprehension:

result = [x + 1 for in range(5)]

medium

A. Missing variable name after 'for'

B. Using + instead of *

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

D. List comprehension cannot use range

Solution

1. Step 1: Check the for loop syntax inside comprehension

   The syntax requires a variable name after 'for', like 'for x in range(5)'. Here, 'x' is missing.

2. Step 2: Identify the error

   Because the variable is missing, Python will raise a syntax error.

3. Final Answer:

   Missing variable name after 'for' -> Option A

4. Quick Check:

   for loop needs variable name [OK]

Hint: Always include variable after 'for' in comprehension [OK]

Common Mistakes:

• Omitting the loop variable
• Misplacing 'in' keyword
• Thinking range needs to start at 1

5. You have a list of words: words = ['apple', 'bee', 'cat', 'dog', 'elephant']. Using list comprehension, how do you create a list of the lengths of words that have more than 3 letters?

hard

A. [w for w in words if len(w) > 3]

B. [len(w) for w in words if len(w) > 3]

C. [len(words) for w in words if len(w) > 3]

D. [len(w) > 3 for w in words]

Solution

1. Step 1: Understand the filtering condition

   We want only words with length greater than 3, so use 'if len(w) > 3' to filter.

2. Step 2: Create list of lengths

   For each filtered word, get its length with len(w). So the comprehension is [len(w) for w in words if len(w) > 3].

3. Final Answer:

   [len(w) for w in words if len(w) > 3] -> Option B

4. Quick Check:

   Filter words by length, then get lengths [OK]

Hint: Filter first, then apply len() in comprehension [OK]

Common Mistakes:

• Returning words instead of lengths
• Using len(words) instead of len(w)
• Returning boolean instead of lengths