What are list comprehensions and when should you use them?

Question

Accepted Answer

A list comprehension is a concise Python expression that builds a new list by iterating over one or more iterables, optionally filtering items. It combines for and if clauses into a single bracketed expression and is typically faster and more readable than the equivalent for -loop-plus- append pattern. General form: [expr for item in iterable if condition] Simple examples: squares = [x * x for x in range(10)] evens = [x for x in range(20) if x % 2 == 0] upper = [name.upper() for name in ["ada", "linus"]] pairs = [(x, y) for x in range(3) for y in range(3) if x != y] Why comprehensions are preferred: Expressiveness — one line communicates intent (“a new list of X for each Y”) more clearly than a loop scaffold. Performance — the loop is pushed into the C implementation of the list type, avoiding repeated attribute lookups of append in Python-level code. Scoping — the loop variable lives inside the comprehension’s own scope (Python 3), so it doesn’t leak into the surrounding namespace. Show all four types: list, dict, set, and generator expressions. The generator expression for memory efficiency (sum without creating a list) is an advanced point. Know when NOT to use comprehensions: complex logic, side effects, more than 2 nested levels.