List Comprehensions

In the For Loops lesson you built new lists by starting with an empty list and appending inside a loop. That pattern is so common that Python has a shortcut for it: the list comprehension. It builds a new list from another collection in a single, readable line.

From loop to comprehension

Here's the loop way of squaring some numbers:

numbers = [1, 2, 3, 4, 5]

squares = []
for n in numbers:
    squares.append(n ** 2)

print(squares)

And here's the same thing as a list comprehension:

numbers = [1, 2, 3, 4, 5]

squares = [n ** 2 for n in numbers]
print(squares)

Read it as: "n ** 2 for each n in numbers". The general shape is:

new_list = [expression for item in collection]

More examples

The expression can be anything: a calculation, a method call or a function call.

names = ["ada", "grace", "alan"]
print([name.capitalize() for name in names])
print([len(name) for name in names])

print([n * 10 for n in range(5)])

prices = [4.99, 12.5, 3.25]
print([round(p * 1.2, 2) for p in prices])   # add 20% tax

Filtering with if

Add if at the end to keep only some items:

new_list = [expression for item in collection if condition]

numbers = [12, 7, 3, 18, 25, 4, 9]

evens = [n for n in numbers if n % 2 == 0]
print(evens)

big = [n for n in numbers if n > 10]
print(big)

words = ["apple", "kiwi", "banana", "fig", "cherry"]
print([w for w in words if len(w) > 4])

Here's the example from the start of this tutorial: word lengths, skipping the word "the":

sentence = "the quick brown fox jumps over the lazy dog"
words = sentence.split()

word_lengths = [len(word) for word in words if word != "the"]
print(words)
print(word_lengths)

Choosing a value with if-else

To transform each item one of two ways, rather than filter, put a conditional expression at the front:

scores = [45, 82, 67, 91, 38]

results = ["pass" if s >= 50 else "fail" for s in scores]
print(results)

numbers = [3, -1, 4, -5, 9]
print([n if n > 0 else 0 for n in numbers])   # replace negatives with 0

Remember: if at the end filters items out. if ... else at the front chooses what each item becomes.

Dictionary comprehensions

Use curly braces and a key: value expression to build a dictionary:

names = ["ada", "grace", "alan"]

name_lengths = {name: len(name) for name in names}
print(name_lengths)

prices = {"coffee": 3.0, "tea": 2.5, "cake": 4.0}
sale = {item: round(price * 0.8, 2) for item, price in prices.items()}
print(sale)

cheap = {item: price for item, price in prices.items() if price < 3.5}
print(cheap)

words = ["Apple", "avocado", "Banana", "blueberry", "cherry"]

first_letters = {w[0].lower() for w in words}
print(sorted(first_letters))

pairs = [(x, y) for x in range(1, 4) for y in ["a", "b"]]
print(pairs)

grid = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [n for row in grid for n in row]
print(flat)

When not to use one

Comprehensions are great for short, clear transformations. If yours gets long, has several conditions or needs print() calls, a regular for loop is easier to read. Readability matters more than saving lines.

Exercises

Exercise 1: Celsius to Fahrenheit

Convert every temperature to Fahrenheit (c * 9 / 5 + 32) with a list comprehension. It should print [32.0, 50.0, 68.0, 86.0, 104.0].

celsius = [0, 10, 20, 30, 40]

Exercise 2: Vowel-starting words

Make a list of the words that start with a vowel. It should print ['apple', 'orange', 'umbrella', 'egg'].

words = ["apple", "banana", "orange", "grape", "umbrella", "egg", "kiwi"]

Exercise 3: Rewrite the loop

Rewrite this loop as a single list comprehension that produces the same result.

numbers = range(1, 21)

result = []
for n in numbers:
    if n % 3 == 0:
        result.append(n * n)

print(result)

Exercise 4: Grade book

Use a dictionary comprehension to map each student to "pass" (score 50 or more) or "fail". It should print {'Mia': 'pass', 'Leo': 'fail', 'Ava': 'pass'}.

scores = {"Mia": 78, "Leo": 42, "Ava": 91}

Summary

• [expr for item in collection] builds a new list in one line.
• Add if condition at the end to filter items.
• Put a if condition else b at the front to choose each item's value.
• {key: value for ...} builds a dictionary the same way.
• Use a regular loop when a comprehension stops being easy to read.