List comprehensions

A list comprehension builds a list in one line:

[expression for item in sequence if condition]

doubled = [n * 2 for n in range(5)]          # [0, 2, 4, 6, 8]
shout   = [w.upper() for w in ["hi", "yo"]]  # ['HI', 'YO']
small   = [n for n in [5, 1, 8] if n < 6]    # [5, 1]

A filter can also use a string: an empty string counts as False, so if r.strip() skips entries that become empty after trimming spaces.

See it run

nums = [1, 2, 3, 4, 5, 6]
big = [n * 10 for n in nums if n > 3]
print(big)

A comprehension is a loop that builds a new list. Read it as: for each n in nums, keep it only if n > 3, and store n 10*.

Practice list comprehensions

Exercise 1

Using comprehensions:

squares = the squares of 1 to 10
evens = the even numbers from 1 to 20

Exercise 2

Use raw in the editor below. With one comprehension, make names: each name stripped of spaces and in title case, skipping entries that are blank. Expected: ["Ana", "Ben", "Cy"].

Exercise 3

Use prices in the editor below. The café is having a 20% off sale. A dict comprehension works like a list one, with curly braces and key: value:

{k: v * 2 for k, v in d.items()}

sale = a new dict with every price 20% off, rounded to 2 decimals
cheap = an alphabetically sorted list of items whose sale price is under 4

The explanation and tasks are public. Running this lesson requires an account with course access.

Practice

Loading Python…

Loading exercise content…

Visualizer Watch your code run line by line

Run your code, then choose Play to follow its execution.

Memory · names → values

Variables appear here as Python creates them.

Output & plots