What is structural pattern matching (match/case)? | Interview Question

Q: What is structural pattern matching (match/case)?

**Target Audience:** Python developers who know `if`/`elif`, lists, dicts and classes and have never used `match`, plus anyone who wants the exact rules an interviewer probes. **Difficulty:** Medium **Python version:** `match` needs Python 3.10+. Behaviour and bytecode checked on CPython 3.11 (gotchas re-run on 3.13); both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Ideas come in plain language first. The words an interviewer expects you to use are tagged `📌 Interview term:` and defined on the spot; all of them are collected in the Quick Glossary at the end. Every output shown was printed by a real run. ## 1. Why This Even Matters — A Story First A post-office sorter does not read every letter. They look at the **shape** of each item: a small flat envelope goes in tray A; a box with a customs label goes to customs, and they note the country written on that label; a parcel marked "fragile" with a weight over 5 kg goes to the special

The problem

Question presented to candidate: "Python 3.10 added match and case. Is it just a switch statement? Walk me through the kinds of patterns you can write, and tell me about a gotcha that catches people out."

What a strong answer should cover:

It is structural: a pattern checks the shape of the subject (type, length, keys, attributes) and binds parts of it to names, not just compares values. Cases are tried top to bottom and the first match wins; no fall-through.
The pattern kinds: literal, capture, wildcard _, value (dotted name), sequence (with *rest), mapping (with **rest), class (Point(x=0), positional via __match_args__), OR |, AS as, plus if guards.
The capture gotcha: a bare name such as case RED: binds anything; constants must be dotted (Color.RED). CPython rejects a bare-name case that is not last with a SyntaxError.
Rules that surprise people: str/bytes never match sequence patterns, mapping patterns ignore extra keys, case 1: matches True (== comparison) while True/False/None compare with is, and no matching case is silently a no-op.
When to use it: parsing commands, tagged JSON-like data, AST and message handling. A dict of handlers or plain if is still better for simple value lookups.

Code / implementation expected: Optional — a short match over mixed inputs using two or three pattern kinds.

match
pattern-matching

Try it yourself

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

Solution

Target Audience: Python developers who know if/elif, lists, dicts and classes and have never used match, plus anyone who wants the exact rules an interviewer probes. Difficulty: Medium Python version: match needs Python 3.10+. Behaviour and bytecode checked on CPython 3.11 (gotchas re-run on 3.13); both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Ideas come in plain language first. The words an interviewer expects you to use are tagged 📌 Interview term: and defined on the spot; all of them are collected in the Quick Glossary at the end. Every output shown was printed by a real run.

1. Why This Even Matters — A Story First

A post-office sorter does not read every letter. They look at the shape of each item: a small flat envelope goes in tray A; a box with a customs label goes to customs, and they note the c

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9 min read · 11 sections

Run the code

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

PythonEvery pattern kind in one dispatcher

Practice

PythonSix gotchas: capture vs constant, str sequences, extra keys, True vs 1

Practice

Discussion

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