What is the walrus operator?

Q: What is the walrus operator?

**Target Audience:** Beginners who have seen `:=` and want to know when it helps; candidates who want the scope and syntax rules. **Difficulty:** Easy **Python version:** Available since Python 3.8. Behaviour checked on CPython 3.11.17 (and 3.13.16, same output) and in the page runner (Pyodide 0.25.1, CPython 3.11.3). > **How to read this doc:** Every idea is explained in plain words, then tagged `📌 Interview term:` with the name an interviewer expects. All output is from real runs, collected in the **Verified** section. ## 1. Why This Even Matters — A Story First You are at a coffee shop counter. The barista calls out "large oat latte!" and, in the same moment, writes your name on the cup. One action, two results: the order is announced *and* recorded. Before the walrus operator, Python could only do the recording as a separate step — write the name on the cup, then hold it up and call it out — which meant either an extra line or doing the work twice. `:=` is "call it out and wri

The problem

Question presented to candidate: "What is the walrus operator, :=? Show me one place where it makes code genuinely better, and one place where you would not use it."

What a strong answer should cover:

:= is an assignment expression (PEP 572, Python 3.8): it binds a name and evaluates to the value, so it can sit inside if, while, comprehensions and calls, where a normal = statement cannot.
The good uses: a read loop while (chunk := f.read(n)):, keeping a regex match you just tested, and computing a value once in a comprehension that both filters on it and keeps it.
Precedence: it binds more loosely than comparisons, so if n := len(a) > 3 stores a boolean; parenthesise (n := len(a)) > 3.
Scope: inside a comprehension the target binds in the enclosing function or module scope (the loop variable does not leak, the walrus target does).
Restrictions: not a bare statement, only plain names as targets (no attributes or subscripts), cannot rebind a comprehension iteration variable, not inside a comprehension in a class body.

Code / implementation expected: Yes — a short before/after, for example a comprehension that calls an expensive function once instead of twice.

walrus

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners who have seen := and want to know when it helps; candidates who want the scope and syntax rules. Difficulty: Easy Python version: Available since Python 3.8. Behaviour checked on CPython 3.11.17 (and 3.13.16, same output) and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

How to read this doc: Every idea is explained in plain words, then tagged 📌 Interview term: with the name an interviewer expects. All output is from real runs, collected in the Verified section.

1. Why This Even Matters — A Story First

You are at a coffee shop counter. The barista calls out "large oat latte!" and, in the same moment, writes your name on the cup. One action, two results: the order is announced and recorded. Before the walrus operator, Python could only do the recording as a separate step — write the name on the cup, then hold it up and c

The solution is hidden so you can try first

9 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonThe good uses: read loops, regex matches, compute-once comprehensions, any() witness