Python Intermediate: Walrus operator

The walrus operator := in Python was introduced in PEP 572. This PEP proposed new syntax to allow assignment to variables within an expression, aiming to make certain patterns of code more concise and readable.

Python Intermediate: Walrus operator

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Python, known for its readability and simplicity, introduced a new feature in version 3.8 that sparked both interest and debate in the programming community: the walrus operator :=. Officially known as assignment expressions and brought into existence by PEP 572, this operator has created new pathways for writing more concise and potentially efficient code. But what exactly is the walrus operator, and how can it be effectively utilized in Python programming?

The background

The primary drive for introducing the walrus operator was to enhance Python’s expressiveness, particularly in situations where assigning a value and using it in an immediate context is required. Before its introduction, programmers often had to use additional lines of code or temporary variables, especially in loops and conditional statements. Let’s consider the following code snippet.

line = file.readline()
while line:
    line = file.readline()

The provided code initiates a process of reading lines from a file, line by line, using a while loop. Initially, the first line of the file is read and assigned to the variable line. The while loop is structured to continue as long as line is not an empty string, indicating that there are more lines to read. Within the loop, each line is printed to the console. Crucially, after printing, the next line is read from the file and assigned to line, ensuring the loop’s progression. This sequence continues until the end of the file is reached, at which point the loop exits. Essentially, the code efficiently prints all lines from the file, handling the process of reading and printing within a concise loop structure.

Walrus operator

At its core, the walrus operator allows us to assign values to variables as part of an expression. It’s a syntactical convenience that combines the processes of assignment and return value in a single, streamlined operation. For example, x := y assigns the value of y to x, and simultaneously x becomes an expression that can be used immediately in a broader context. Now let’s see how the code changes when we use the operator.

while (line := file.readline()):

The provided code utilizes the walrus operator to succinctly read lines from a file and print them within a while loop. The condition of the loop is set to (line := file.readline()), where the walrus operator simultaneously assigns the value of the next line from the file to the variable line and checks if it’s not an empty string. As long as there are lines to be read, the loop continues. Inside the loop, each line is printed to the console using print(line). This concise structure elegantly combines the assignment and condition check, resulting in a streamlined way to iterate through the lines of the file and print them until the end of the file is reached.

More examples

The walrus operator := is useful in several scenarios where it can make code more concise and readable. Here are some common use cases.

In Conditional Expressions: We can use it within an if statement to assign a value to a variable as part of the condition.

if (n := len(a)) > 10:
    print(f"List has {n} elements")

List Comprehensions: It allows assignments within list comprehensions, making them more powerful and avoiding external initializations.

[y for x in data if (y := process(x)) is not None]

Avoiding Redundant Function Calls: If a function call is expensive and we need the result in multiple places within a block, we can use the walrus operator to call it once and use the result multiple times.

if (result := expensive_computation()) > threshold:

Working with Regular Expressions: When extracting matches from a regular expression, it can simplify the process by combining the match operation and the condition check.

if (match :=, string)):

Streamlining Data Processing: In data processing tasks, it can be used for conditionally processing elements.

processed = [process(item) for item in data if (check := condition(item))]

These use cases highlight the walrus operator’s ability to make certain patterns of code more concise, reducing the verbosity of Python code, especially in situations that involve repeated evaluation of an expression.


The walrus operator in Python is a testament to the language’s evolving nature, aimed at providing programmers with tools for more expressive coding. While its adoption requires careful consideration to maintain the readability that Python is renowned for, it undoubtedly offers a powerful tool for writing succinct and efficient code. As with any feature, its true value comes from judicious and thoughtful application, enhancing Python’s capability to elegantly solve a wide array of programming challenges.

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PEP 572 – Assignment Expressions |
Python Enhancement Proposals (PEPs)