Python ยท Chapter 23 of 45
List Comprehension
A list comprehension builds a new list from an iterable in a single readable line.
Syntax: `[expression for item in iterable if condition]`.
Basic form
Replaces short `for` loops that append to a list. Faster and clearer for simple transformations.
With filter
Add `if condition` after the loop to keep only certain items.
Example 1 (python)
squares = [x*x for x in range(1, 6)]
print(squares)Output
[1, 4, 9, 16, 25]Build a list of squares 1..5.
Example 2 (python)
evens = [n for n in range(10) if n % 2 == 0]
print(evens)Output
[0, 2, 4, 6, 8]Filter with an if clause.
Key points
- Compact way to build lists.
- Also works for sets `{...}` and dicts `{k:v for ...}`.
- Add `if` to filter.
- Prefer a normal loop when logic is complex.
๐ก Note: Nested comprehensions read left-to-right in the same order as nested `for` loops.