PythonLesson 12 of 16

List comprehensions in Python

A list comprehension is a short way to build a new list from an old one. What usually takes a loop and append fits on a single line.

Key points

[expression for x in items] makes a list of transformed items
[x for x in items if condition] keeps only the matching ones
The original list stays untouched
sum(), min(), max() and len() work nicely on the result
If the line gets hard to read, go back to a normal loop

Example

squares = [n * n for n in range(1, 6)]print(squares) names = ["anna", "leo", "mia"]print([name.title() for name in names]) prices = [90, 450, 120, 700]cheap = [p for p in prices if p < 200]print(cheap)

Three comprehensions: squares, names with a capital letter, and only the cheap prices. Each one replaces a three-line loop.

Task

How to use the editor⌘ Enter or Ctrl Enter runs the code, ⇧ ⌘ Enter checks the task.Tab and Shift Tab indent the line or the selection; Esc, then Tab leaves the editor.The check compares what the program printed with the task; spaces at line ends do not matter.After two failed checks you may open the solution. Your code is kept in this browser.

The weather log has five temperatures. Rewrite warm as a list comprehension that keeps only days of 20 degrees or more. The average works itself out.

temps = [18, 23, 25, 16, 21]# TODO: rewrite warm as a list comprehension that keeps days of 20 degrees or morewarm = tempsprint("Warm days:", warm)print("Average:", sum(warm) / len(warm))

temps = [18, 23, 25, 16, 21]
# TODO: rewrite warm as a list comprehension that keeps days of 20 degrees or more
warm = temps
print("Warm days:", warm)
print("Average:", sum(warm) / len(warm))

Output

What your program prints appears here.