Walrus Operator (:=)

The walrus operator (assignment expression) assigns a value and returns it in a single expression. Introduced in Python 3.8 (PEP 572), it enables more concise code in loops, conditions, and comprehensions.

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Basic Usage

Assignment in Condition

if (n := len("hello")) > 3:
    print(f"String length {n} is greater than 3")

Output:

String length 5 is greater than 3

Assignment in while Loop

data = iter([1, 2, 3, None, 4])

while (value := next(data, None)) is not None:
    print(f"Value: {value}")

Output:

Value: 1
Value: 2
Value: 3

List Comprehension Usage

Filtering with Walrus

numbers = range(10)
squared = [y for x in numbers if (y := x**2) > 20]
print(squared)

Output:

[25, 36, 49, 64, 81]

Reusing Computation

import re

text = "Hello123World"
pattern = re.compile(r'\d+')

if (match := pattern.search(text)):
    print(f"Found: {match.group()}")

Output:

Found: 123

Practical Examples

While Loop with Input

import io

user_input = iter(["valid", "another", "quit"])

while (command := next(user_input)) != "quit":
    print(f"Processing: {command}")

Output:

Processing: valid
Processing: another

Dictionary Comprehension

data = ["apple", "pie", "a", "banana"]
lengths = {word: length for word in data if (length := len(word)) > 2}
print(lengths)

Output:

{'apple': 5, 'banana': 6}

Advantages

Cleaner Code

def some_expensive_computation():
    return [1, 2, 3]

def process(x):
    pass

if (data := some_expensive_computation()):
    process(data)

Output: