List Comprehensions in Python

List comprehensions provide a concise way to construct lists from iterables, combining mapping and filtering in a single expression.

Basic form

[expr for item in iterable]

squares = [n*n for n in range(6)]

Add an if clause to keep only items matching a condition.

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

Apply functions or expressions.

upper = [s.upper() for s in ["a", "b", "c"]]
lengths = [len(s) for s in ["py", "python"]]

Comprehensions execute left‑to‑right; use multiple for clauses to flatten.

grid = [[1,2,3], [4,5,6]]
flat = [x for row in grid for x in row]  # [1,2,3,4,5,6]

Use a ternary in the expression part for transforms that depend on a condition.

labels = ["even" if n % 2 == 0 else "odd" for n in range(5)]

index = {s: i for i, s in enumerate(["a","b"])}
uniq = {c for c in "hello"}

Use parentheses to create a generator for streaming.

total = sum(n*n for n in range(10))

Avoid deeply nested comprehensions—prefer named loops for readability
Don’t leak loop variables in 3.x (comprehension variables are local to the expression)