Walrus Operator (:=) in Python

The walrus operator (:=) is an assignment expression introduced in Python 3.8. It allows you to assign a value to a variable as part of an expression, making your code more concise and readable.

Syntax

variable := expression

Here, variable is assigned the value of expression, and the whole expression evaluates to the value.

Examples of Walrus Operator Usage

1. Using in if Condition

Instead of:

value = len(my_list)
if value > 5:
    print(f"List is long: {value}")

We can use:

my_list = [1, 2, 3, 4, 5, 6, 7]
if (value := len(my_list)) > 5:
    print(f"List is long: {value}")

Why? It avoids writing len(my_list) twice, making it more efficient.

2. Using in while Loops

Instead of:

data = input("Enter something: ")
while len(data) > 0:
    print(f"You entered: {data}")
    data = input("Enter something: ")

We can use:

while (data := input("Enter something: ")):
    print(f"You entered: {data}")

Why? This avoids calling input() twice, making the loop cleaner.

3. Using in List Comprehensions

Instead of:

numbers = [10, 15, 20, 25, 30]
filtered = []
for num in numbers:
    square = num ** 2
    if square > 400:
        filtered.append(square)

We can use:

numbers = [10, 15, 20, 25, 30]
filtered = [square for num in numbers if (square := num ** 2) > 400]
print(filtered)  # Output: [625, 900]

Why? The walrus operator lets us compute the square once instead of repeating it.

4. Using in match Statements (Python 3.10+)

def categorize(number):
    match number:
        case n if (n := number) > 50:
            print(f"Large number: {n}")
        case _:
            print(f"Small number: {n}")

categorize(60)  # Output: Large number: 60

Why? We assign n inside case n if ..., making the code cleaner.

Real-Life Usage of Walrus Operator

1. Efficient Logging:

   import logging
   if (msg := "Error occurred!") and is_critical:
       logging.error(msg)
   

   Why? The message is assigned and logged in one step.

2. Optimized API Calls:

   if (response := fetch_data()).status_code == 200:
       print(response.json())
   

   Why? It avoids making multiple API calls.

3. Reading Files Line by Line Efficiently:

   with open("data.txt") as file:
       while (line := file.readline().strip()):
           print(line)
   

   Why? It prevents extra file reads.

Tasks Using the Walrus Operator

Task 1: Count User Inputs Until “exit”

Write a Python program that takes user input and counts how many times a user enters something before typing “exit”.

Solution:

count = 0
while (user_input := input("Enter something (or 'exit' to stop): ")) != "exit":
    count += 1
print(f"You entered {count} times before exiting.")

Task 2: Find the First Long Word in a List

Given a list of words, find and print the first word that has more than 7 letters.

words = ["apple", "banana", "strawberry", "kiwi", "pineapple"]
if (long_word := next((word for word in words if len(word) > 7), None)):
    print(f"First long word: {long_word}")

Why? This finds the first long word efficiently.

Task 3: Sum Numbers Until a Negative Number

Write a program that keeps taking numbers from the user and sums them until the user enters a negative number.

total = 0
while (num := int(input("Enter a number: "))) >= 0:
    total += num
print(f"Total sum: {total}")

Why? This makes the code more compact.

Conclusion

The walrus operator (:=) improves readability and efficiency by combining assignment and expressions in a single step. However, use it wisely—overusing it can make code harder to read.