Python walrus operator (Python 3.8 assignment expression)

The “Python walrus operator”, officially known as the assignment expression operator, was introduced in Python 3.8. It’s symbolized by a colon followed by an equal sign :=.

The Python community refers to it as the “walrus operator” due to its resemblance to a pair of eyes and tusks, like that of a walrus.

 

 

The Need for the Walrus Operator

Before the introduction of the walrus operator, Python developers had to assign values to variables in one line and use them in comparisons in the next.

This often resulted in multiple lines of code for simple operations.

# Python code without using the walrus operator
value = 10
if value > 5:
    print(value)

Output

10

In this code snippet, we had to write two lines to assign the value and then use it for comparison.

The walrus operator allows developers to assign and use variables within the same expression.

 

Syntax of the Walrus Operator

The syntax of the walrus operator is relatively straightforward. It involves a variable, a walrus operator :=, and an expression.

Remember to enclose the assignment expression in parentheses.

# Using the walrus operator to assign a value and return it
(value := 10)

Output

10

In this case, the walrus operator assigns the value 10 to value and also returns the value 10. However, remember that you can’t use this operator in a stand-alone statement, unlike the standard assignment operator =.

 

Using in If Statements and While Loops

The walrus operator can be used in if statements and while loops to make the code more concise. Here’s how to use the walrus operator in if statements.

input_value = "Hello"
if (value := len(input_value)) > 4:
    print(f"The length of the string is {value}")

Output:

The length of the string is 5

In this example, the walrus operator is used to assign the length of input_value to value and compare it with 4 in the same line.

As the length of the string “Hello” is 5, which is greater than 4, the message is printed.

You can also use the walrus operator in while loops to make the code more concise:

# Reading a user input until a non-empty string is entered
while (value := input("Enter a non-empty string: ")) == "":
    print("The string is empty.")
print(f"The non-empty string is '{value}'")

Output:

Enter a non-empty string:
The string is empty.
Enter a non-empty string: Hello
The non-empty string is 'Hello'

In this example, the walrus operator is used to assign the value of input("Enter a non-empty string: ") to value and compare it to an empty string "".

The loop continues to execute as long as the user enters an empty string.

As soon as a non-empty string is entered, it breaks out of the loop and prints the non-empty string.

 

Using in List Comprehensions

The walrus operator is handy when working with list comprehensions in Python. This allows for more complex calculations within list comprehensions without calling a function multiple times.

import random
numbers = [number for _ in range(10) if (number := random.randint(1, 20)) % 2 == 0]
print(numbers)

Output

[10, 14, 6, 16, 10]

In this example, the walrus operator assigns the value of random.randint(1, 20) to number and checks if it’s even. If it is, it adds the number to the list. This results in a list of random even numbers.

 

Walrus Operator with Data Structures

The walrus operator can be effectively used with Python’s data structures like lists, sets, and dictionaries, as well as in comprehensions for these data structures.

# Using walrus operator with a list in a dictionary
numbers = [5, 3, 7, 1, 2, 8, 6, 4]
description = {
    "max": (num_max := max(numbers)),
    "min": (num_min := min(numbers)),
}
print(description)

Output:

{
    'max': 8,
    'min': 1
}

In this example, the walrus operator is used to assign the maximum and minimum values of the numbers list to the variables num_max and num_min within the dictionary comprehension.

 

When to Use and When Not

While the walrus operator offers many advantages, it should be used judiciously. It’s best suited to situations where using it can make the code more concise without sacrificing readability.

For example, it’s beneficial when a variable needs to be assigned and used in the same line, such as within conditions or list comprehensions.
On the other hand, it may not be suitable for complex expressions, as it can make the code difficult to read and understand.

Similarly, in situations where a stand-alone assignment is needed, the traditional assignment operator = should be used instead of the walrus operator.

 

Compatibility and Version Support

The walrus operator is a new operator introduced in Python 3.8. As such, it is not available in Python versions prior to 3.8.

For new projects or projects that are guaranteed to run on Python 3.8 or later, feel free to use the walrus operator whenever it improves your code’s clarity and conciseness.

If you’re working on a project that needs to support older Python versions, you should avoid using the walrus operator or rewrite your code in the new syntax.

 

Tips for Transitioning from Traditional Python Syntax

Transitioning to use the walrus operator from traditional Python syntax can be straightforward with the following tips:

1. Start using the walrus operator in simple use cases like if conditions or while loops.
2. Gradually move to more advanced uses like list comprehensions and function calls.
3. Always consider the readability of your code. If the use of the walrus operator makes your code hard to read, it might be better to stick with traditional syntax.

 

Resource:https://docs.python.org/3/whatsnew/3.8.html