Python List Comprehensions Explained

A list comprehension is a compact way to build a new list by applying an expression to every item in an iterable, optionally filtering with a condition.

squares = [x * x for x in range(10) if x % 2 == 0]
# [0, 4, 16, 36, 64]

Steps

1. Write the expression to apply to each item first: x * x
2. Follow it with 'for x in range(10)' to define the source iterable
3. Optionally add 'if x % 2 == 0' at the end to filter which items are included

How it works

List comprehensions are equivalent to a for-loop that appends to a list, but they're generally faster in CPython because the looping happens at the C level rather than through repeated Python-level append() calls.

Things to watch for

• Nesting more than one level of comprehension (or adding multiple conditions) quickly hurts readability — fall back to a regular loop when that happens
• A generator expression (x*x for x in range(10)) uses the same syntax with () instead of [] and is more memory-efficient when you don't need the full list at once

FAQ

When should I use a for-loop instead of a list comprehension?

When the loop body has side effects (like printing or writing to a file) or spans multiple statements — comprehensions are meant for building a list from an expression, not for general-purpose looping.