9.5 List comprehensions

Learning objectives

By the end of this section you should be able to

Identify the different components of a list comprehension statement.
Implement filtering using list comprehension.

List comprehensions

A list comprehension is a Python statement to compactly create a new list using a pattern.

The general form of a list comprehension statement is shown below.

list_name = [expression for loop_variable in iterable]

list_name refers to the name of a new list, which can be anything, and the for is the for loop keyword. An expression defines what will become part of the new list. loop_variable is an iterator, and iterable is an object that can be iterated, such as a list or string.

Example 1 Creating a new list with a list comprehension

A list comprehension shown below in the second code has the same effect as the regular for loop shown in the first code. The resultant list is [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] in both cases.

Creating a list of squares using a for loop.

# Create an empty List.
squares_list = []

# Add items to a list, as squares of numbers starting at 0 and ending at 9.
for i in range(10):
  squares_list.append(i*i)

Creating a list of squares using the list comprehension.

square_list = [i*i for i in range(10)]

The expression i*i is applied for each value of the loop_variable i.

Example 2 A Dr. Seuss poem

A list comprehension can be used to create a list based on another list. In line 6, the for loop is written on the list poem_lines.

# Create a list of words
words_list = ["one", "two", "red", "blue"]

# Use a list comprehension to create a new list called poem_lines
# Inserting the word "fish" attached to each word in words_list
poem_lines = [w + " fish" for w in words_list]
for line in poem_lines:
  print(line)

The above code's output is:


    one fish
    two fish
    red fish
    blue fish

Filtering using list comprehensions

List comprehensions can be used to filter items from a given list. A condition is added to the list comprehension.

list_name = [expression for loop_variable in container if condition]

In a filter list comprehension, an element is added into list_name only if the condition is met.

Note

Selecting five-letter words

Write a program that creates a list of only five-letter words from the given list and prints the new list.

# A list
word_list = ["notebook", "capsule", "ruler", "divider", "compass", "chart"]

Note

Books starting with "A"

Write a program that selects words that begin with an "A" in the given list. Make sure the new list is then sorted in dictionary order. Finally, print the new sorted list.

# A list of book titles
book_list = ["Pachinko", "A Hitchhiker's Guide to the Galaxy", "Moby Dick", "Beloved", "A Farewell to Arms", "Anna Karenina", "The Golden Notebook"]