Write a List in One Line (List Comprehensions)

Source: Dev.to

Introduction

You can build lists in Python with a loop, but list comprehensions let you do the same work in a single, readable line.

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
squares = []

for n in numbers:
    squares.append(n * n)

print(squares)
# Output: [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

The same result with a list comprehension:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
squares = [n * n for n in numbers]

print(squares)
# Output: [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Syntax

Read a comprehension left‑to‑right like a sentence:

[expression for variable in iterable]

• expression – what you want in the new list (calculation, function call, etc.).
• variable – the loop variable (e.g., n).
• iterable – any iterable object (list, range, string, etc.).

Adding a filter

[expression for variable in iterable if condition]

The if clause keeps only items where condition is True.

Examples

Simple transformations

names = ["alex", "priya", "sam", "jordan"]
upper_names = [name.upper() for name in names]
print(upper_names)
# Output: ['ALEX', 'PRIYA', 'SAM', 'JORDAN']

lengths = [len(name) for name in names]
print(lengths)
# Output: [4, 5, 3, 6]

numbers = range(1, 6)
doubled = [n * 2 for n in numbers]
print(doubled)
# Output: [2, 4, 6, 8, 10]

Filtering

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = [n for n in numbers if n % 2 == 0]
print(evens)
# Output: [2, 4, 6, 8, 10]

scores = [45, 92, 78, 61, 34, 88, 55, 97]
passing = [s for s in scores if s >= 60]
print(passing)
# Output: [92, 78, 61, 88, 97]

Transform and filter together

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
squared_evens = [n * n for n in numbers if n % 2 == 0]
print(squared_evens)
# Output: [4, 16, 36, 64, 100]

Working with strings

sentence = "Hello World"
letters = [char for char in sentence if char != " "]
print(letters)
# Output: ['H', 'e', 'l', 'l', 'o', 'W', 'o', 'r', 'l', 'd']

words = ["  hello  ", "  world  ", "  python  "]
cleaned = [w.strip() for w in words]
print(cleaned)
# Output: ['hello', 'world', 'python']

Dictionary comprehensions

names = ["Alex", "Priya", "Sam"]
name_lengths = {name: len(name) for name in names}
print(name_lengths)
# Output: {'Alex': 4, 'Priya': 5, 'Sam': 3}

scores = {"Alex": 92, "Priya": 58, "Sam": 75, "Jordan": 44}
passing = {name: score for name, score in scores.items() if score >= 60}
print(passing)
# Output: {'Alex': 92, 'Sam': 75}

When Not to Use a Comprehension

If the logic becomes complex, a regular loop is clearer:

# Hard to read comprehension
result = [process(transform(validate(x))) for x in data if check_one(x) and check_two(x)]

# Clearer as a loop
result = []
for x in data:
    if check_one(x) and check_two(x):
        validated = validate(x)
        transformed = transform(validated)
        result.append(process(transformed))

Rule of thumb: use a comprehension only when it can be understood at a glance.

Practice Exercise

Create a file comprehensions_practice.py and, using only comprehensions, perform the following tasks with the given data:

temperatures_c = [0, 10, 20, 30, 40, 100]
words = ["Python", "is", "great", "for", "AI", "and", "ML"]
students = [
    {"name": "Alex", "score": 88},
    {"name": "Priya", "score": 52},
    {"name": "Sam", "score": 76},
    {"name": "Jordan", "score": 91},
    {"name": "Lisa", "score": 43}
]

1. Convert each temperature to Fahrenheit (F = C * 9/5 + 32) and store the results in a list.
2. Build a list of words longer than three characters.
3. Extract a list of just the student names.
4. Extract a list of names of students who passed (score ≥ 60).
5. Build a dictionary mapping each student’s name to their score.