Python walrus operator

last modified January 29, 2024

Python walrus operator tutorial shows how to use walrus operator in Python.

Python 3.8 introduced a new walrus operator :=. The name of the operator comes from the fact that is resembles eyes and tusks of a walrus of its side.

The walrus operator creates an assignment expression. The operator allows us to assign a value to a variable inside a Python expression. It is a convenient operator which makes our code more compact.

print(is_new := True)

We can assign and print a variable in one go.

is_new = True
print(is_new)

Without the walrus operator, we have to create two lines.

Python walrus read input

In the following example, we use the walrus operator in a while loop.

read_words.py

#!/usr/bin/python

words = []

while (word := input("Enter word: ")) != "quit":
    words.append(word)

print(words)

We ask the user to write words, which are appended to a list.

$ ./read_words.py
Enter word: cloud
Enter word: falcon
Enter word: rock
Enter word: quit
['cloud', 'falcon', 'rock']

Python walrus with if condition

Suppose that all our words must have at least three characters.

test_length.py

#!/usr/bin/python

words = ['falcon', 'sky', 'ab', 'water', 'a', 'forest']

for word in words:
    if ((n := len(word)) < 3):
        print(f'warning, the word {word} has {n} characters')

In the example, we use the walrus operator to test the length of a word. If a word has less than three characters, a warning is issued. We determine and assign the length of a word in one shot.

$ ./test_length.py
warning, the word ab has 2 characters
warning, the word a has 1 characters

Python walrus reading file

In the next example, we use the walrus operator to read a file.

words.txt

falcon
sky
cloud
water
rock
forest

We have some words in the words.txt file.

read_file.py

#!/usr/bin/python

with open('words.txt', 'r') as f:

    while line := f.readline():

        print(line.rstrip())

The example reads the file using the readline method. The walrus operator makes the code shorter.

Python walrus traverse container

In the following example, we use the walrus operator when traversing a list of dictionaries.

traversing.py

#!/usr/bin/python

users = [ 
    {'name': 'John Doe', 'occupation': 'gardener'},
    {'name': None, 'occupation': 'teacher'}, 
    {'name': 'Robert Brown', 'occupation': 'driver'}, 
    {'name': None, 'occupation': 'driver'}, 
    {'name': 'Marta Newt', 'occupation': 'journalist'} 
] 
  
for user in users:  
    if ((name := user.get('name')) is not None): 
        print(f'{name} is a {user.get("occupation")}')

In the example, we have None values in the dictionaries. We print all users who have name specified.

$ ./traversing.py
John Doe is a gardener
Robert Brown is a driver
Marta Newt is a journalist

There are three users who have their names specified.

Python walrus with regex

In the following example, we use the walrus operator in a regular expression.

search.py

#!/usr/bin/python

import re

data = 'There is a book on the table.'

pattern = re.compile(r'book')

if match := pattern.search(data):
    print(f'The word {pattern.pattern} is at {match.start(), match.end()}') 
else:
    print(f'No {pattern.pattern} found')

We search for a pattern and assign the match (if found) to a variable in one go.

$ ./search.py
The word book is at (11, 15)

The word book was found at the given indexes.

Source

The Python Language Reference

In this article we have worked with the Python walrus operator.

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My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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