List Comprehensions

A huge share of loops in real Python code follow the same shape: start with an empty list, loop over something, optionally filter, append a transformed value. List comprehensions collapse that pattern into a single line.

The manual version

squares = []
for n in range(10):
    squares.append(n ** 2)

The comprehension version

squares = [n ** 2 for n in range(10)]

Read it as: "n squared, for each n in range(10)" — the expression to build comes first, followed by the loop that drives it. Both versions above produce the exact same list; the comprehension is just a more direct way to say it.

Adding a filter

A comprehension can include an if clause to only include items that match a condition:

evens = [n for n in range(20) if n % 2 == 0]

Compare to the manual loop it replaces:

evens = []
for n in range(20):
    if n % 2 == 0:
        evens.append(n)

Transforming while filtering

The two combine naturally — transform an item, but only for the ones that pass a check:

words = ["apple", "sky", "banana", "ok"]
long_words_upper = [w.upper() for w in words if len(w) > 3]
print(long_words_upper)   # ['APPLE', 'BANANA']

Dictionary and set comprehensions

The same syntax works for building a dict or set instead of a list:

squares_dict = {n: n ** 2 for n in range(5)}
print(squares_dict)   # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

unique_lengths = {len(w) for w in words}

A dict comprehension needs a key: value pair before the for; a set comprehension looks just like a list comprehension but with {} instead of [].

When a comprehension is the wrong choice

Comprehensions are meant to make code more readable, not less. Once the logic needs multiple conditions, nested loops, or a transformation too complex to read in one line, a regular for loop is the better choice:

# Avoid -- hard to read at a glance
result = [x * y for x in range(10) for y in range(10) if x != y if (x + y) % 2 == 0]

# Prefer -- a plain loop, even though it's longer
result = []
for x in range(10):
    for y in range(10):
        if x != y and (x + y) % 2 == 0:
            result.append(x * y)

A good rule of thumb: if you can read the comprehension out loud and it still makes sense as a sentence, keep it. If you have to trace through it mentally like a puzzle, unroll it back into a loop.