4. Advanced Tools · lesson 10 of 17 · 1 min · January 1, 2024

List Comprehensions

Transforming lists the Pythonic way.

Frequently, you’ll want to transform a list into another list, by choosing only certain elements, or by transforming elements, or both. The Pythonic way of doing this is list comprehensions:

comprehensions.py

even_numbers = [x for x in range(5) if x % 2 == 0]   # [0, 2, 4]
squares      = [x * x for x in range(5)]            # [0, 1, 4, 9, 16]
even_squares = [x * x for x in even_numbers]        # [0, 4, 16]

You can similarly turn lists into dictionaries or sets:

dict_set_comprehensions.py

square_dict = { x : x * x for x in range(5) }  # { 0:0, 1:1, 2:4, 3:9, 4:16 }
square_set  = { x * x for x in [1, -1] }       # { 1 }

If you don’t need the value from the list, it’s conventional to use an underscore as the variable:

underscore.py

zeroes = [0 for _ in even_numbers]      # has the same length as even_numbers

A list comprehension can include multiple fors:

multiple_for.py

pairs = [(x, y)
       for x in range(10)
       for y in range(10)]   # 100 pairs (0,0) (0,1) ... (9,8), (9,9)

and later fors can use the results of earlier ones:

dependent_for.py

increasing_pairs = [(x, y)                       # only pairs with x < y,
                  for x in range(10)           # range(lo, hi) equals
                  for y in range(x + 1, 10)]   # [lo, lo + 1, ..., hi - 1]

We will use list comprehensions a lot.