Python - Comprehensions

Overview

Estimated time: 25–35 minutes

Use comprehensions to build lists, dicts, and sets concisely. Learn when they improve clarity and when a loop is clearer.

Learning Objectives

• Write list, dict, and set comprehensions.
• Use conditions in comprehensions.
• Apply readability guidelines for nested comprehensions.

Prerequisites

• Python - Dicts & Sets

Examples

squares = [x*x for x in range(5)]
print(squares)

even_squares = [x*x for x in range(10) if x % 2 == 0]
print(even_squares)

index_map = {ch: i for i, ch in enumerate("abc")}
print(index_map)

uniq = {x % 3 for x in range(10)}
print(uniq)

Expected Output:

[0, 1, 4, 9, 16]
[0, 4, 16, 36, 64]
{'a': 0, 'b': 1, 'c': 2}
{0, 1, 2}

Common Pitfalls

• Overly complex comprehensions. Prefer a for-loop when nesting is deep.
• Confusing the order of for/if clauses; read left-to-right carefully.

Checks for Understanding

1. How do you filter items inside a list comprehension?
2. When should you avoid nested comprehensions?

Exercises

1. Create a dict mapping each word in a sentence to its length using a comprehension.
2. Build a set of unique first letters from a list of names.