5.30. Comprehensions

A comprehension builds a new list, set, dict, or generator from an existing iterable, in a single expression. It is a replacement for the common pattern of starting with an empty container and appending in a loop.

5.30.1. List comprehensions

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

Output:

[0, 1, 4, 9, 16]

The same loop spelled out:

squares = []
for x in range(5):
    squares.append(x * x)

The comprehension form is one expression that builds the list in place. There is no squares = [] and no .append – the result is the value of the comprehension.

The leading expression produces each item; the for clause names the loop variable; an optional if filters items out.

5.30.2. Filtering with if

An optional if clause keeps only the items that match:

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

Output:

[0, 2, 4, 6, 8]

The filter runs before the leading expression – x % 2 == 0 is checked first; only matching values reach x for the output.

5.30.3. Dict and set comprehensions

The same shape works with dict and set literals.

A dict comprehension has a key: value pair before the for:

squares = {x: x * x for x in range(5)}
print(squares)

Output:

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

A set comprehension uses braces and a single expression:

unique_lengths = {len(w) for w in ["a", "bb", "c", "bb"]}
print(unique_lengths)

Output:

{1, 2}

5.30.4. Generator expressions

Round brackets produce a generator expression instead of a list. The values are computed one at a time, on demand:

total = sum(x * x for x in range(1000))

No million-item list is ever built. The values flow one by one into sum(), which adds them and discards each as it goes.

Generator expressions are the right choice when feeding values into a reducing function (sum(), max(), any(), all()) or any other iterator-consuming code – they save the memory the equivalent list would have used.

5.30.5. When not to use a comprehension

Comprehensions are concise but not always clearer. Reach for a plain for loop when:

• The body needs more than one statement (a comprehension fits exactly one expression).

• The body has side effects (printing, writing to a file) – comprehensions are for building a collection, not for running actions.

• The filter or transformation has so many parts that the comprehension no longer reads left-to-right.