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12 · Comprehensions

Part 2 — Data Structures · Estimated time: 25–35 min · Prerequisite: 09 · Lists

A comprehension is a compact, readable way to build a list (or dict or set) from another collection. Anywhere you'd write a loop that creates an empty list and appends to it, a comprehension often does the same job in one clear line. This is one of Python's most loved features.

What you'll learn

  • Turning a build-a-list loop into a list comprehension
  • Filtering items with an if inside a comprehension
  • Dict and set comprehensions

1. List comprehensions

Compare the two ways to build a list of squares:

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

# The long way:
squares = []
for n in nums:
    squares.append(n * n)

# The comprehension — same result, one line:
squares = [n * n for n in nums]
print(squares)   # [1, 4, 9, 16, 25]

Read it left to right: "n * n for each n in nums." The pattern is [expression for item in iterable].

▶️ Run it: python examples/01_list_comprehension.py

2. Filtering with if

Add an if at the end to keep only the items you want.

nums = [1, 2, 3, 4, 5, 6, 7, 8]

evens = [n for n in nums if n % 2 == 0]
print(evens)        # [2, 4, 6, 8]

# Transform and filter at the same time:
big_squares = [n * n for n in nums if n > 5]
print(big_squares)  # [36, 49, 64]

The pattern is [expression for item in iterable if condition].

▶️ Run it: python examples/02_filtering.py

💡 Try it yourself: from ["hi", "hello", "hey", "yo"], build a list of only the words longer than 2 letters.

3. Dict and set comprehensions

The same idea works with {} to build dictionaries and sets.

words = ["apple", "banana", "cherry"]

# Dict comprehension: build key: value pairs.
lengths = {w: len(w) for w in words}
print(lengths)   # {'apple': 5, 'banana': 6, 'cherry': 6}

# Set comprehension: unique results.
nums = [1, 2, 2, 3, 3, 3]
squares = {n * n for n in nums}
print(sorted(squares))   # [1, 4, 9]

▶️ Run it: python examples/03_dict_and_set_comprehensions.py

Common mistakes

Mistake What happens Fix
Cramming complex logic in unreadable one-liner If it needs more than a simple if, use a normal loop.
Using () and expecting a tuple you get a generator, not a tuple Use [] for a list; tuple(...) if you really want a tuple.
Forgetting the if goes at the end SyntaxError Order is [expr for x in xs if cond].
Putting : in a list comprehension that's dict syntax Use key: value only inside {}.

Recap / cheat-sheet

[expr for x in xs]              # list comprehension
[expr for x in xs if cond]     # with a filter
{k: v for x in xs}             # dict comprehension
{expr for x in xs}             # set comprehension (unique results)

# Equivalent loop for the first line:
out = []
for x in xs:
    out.append(expr)

Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

  1. exercises/01_squares.py — build a list of squares.
  2. exercises/02_even_numbers.py — filter out the even numbers.
  3. exercises/03_word_lengths.py — map each word to its length with a dict comprehension.

Try each yourself before opening the solution.

Next → 13 · Functions (coming soon)