Alfred Test Python List Comprehensions in 5 Minutes

A list comprehension builds a new list by applying an expression to each item of an iterable, optionally keeping only the items that pass a condition.

squares = [n * n for n in range(10)]
even_squares = [n * n for n in range(10) if n % 2 == 0]

The same result with a loop takes four lines:

even_squares = []
for n in range(10):
    if n % 2 == 0:
        even_squares.append(n * n)

• Transforming data: cleaning strings, converting types, extracting fields from records.
• Filtering: keeping only the items that match a condition.
• Building lookups: the same syntax works for dicts and sets, e.g. {user.id: user for user in users}.

Switch back to a plain loop when the comprehension needs several conditions or nested loops, when each step has side effects such as logging or writing files, or when a reader would have to stop and decode it. Readability is the point: a comprehension that has to be deciphered has lost its advantage.