How do you enforce match/case exhaustiveness in Python?

Q: How do you enforce match/case exhaustiveness in Python?

**Target Audience:** Python developers who use `match` or `if`/`elif` chains over enums or tagged types and want missing cases caught automatically. **Difficulty:** Medium **Python version:** `assert_never` needs Python 3.11+ (or `typing_extensions`). Runtime behaviour checked on CPython 3.11; both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). Checker output is from mypy 2.3.1 and pyright 1.1.414. > **How to read this doc:** Ideas come in plain language first. Terms an interviewer expects are tagged `📌 Interview term:` and defined on the spot; all of them are collected in the Quick Glossary at the end. Every output shown — Python and type checker — was printed by a real run. ## 1. Why This Even Matters — A Story First A hotel has a checklist for every room type: single, double, suite. Each night the night manager walks the list. One day the owner adds a fourth type, "family room" — but nobody updates the checklist. Nothing breaks. No alarm sounds. Family rooms a

The problem

Question presented to candidate: "You have a match over an order-status enum. Next month someone adds a new status. How do you make sure every match that should handle it gets flagged, instead of silently doing nothing?"

What a strong answer should cover:

Python does not check exhaustiveness at runtime: when no case matches, the match does nothing and a function falls off the end returning None.
The runtime safety net: a final case _: that raises; typing.assert_never(x) (3.11+) does that and also raises AssertionError even under python -O.
The static check: with assert_never(x) in the default branch, mypy and pyright narrow x case by case and report the leftover type (for example Literal[Status.REFUNDED]) when a case is missing.
Checkers can also flag a missing case without assert_never: mypy's opt-in --enable-error-code exhaustive-match (mypy 1.17+) and pyright's reportMatchNotExhaustive (on in strict mode).
It only works over closed types: enums, Literal[...], bool, and unions of classes. A bare object, str or int can never be exhausted.
A test that iterates Enum members or typing.get_args(Union) catches gaps in projects without a checker in CI.

Clarifying questions expected:

Does the project run mypy or pyright in CI, and in which mode?
Is the value a closed set (enum, Literal, union of classes), or arbitrary input that needs a real default?

Code / implementation expected: Yes — a match ending in case _: assert_never(x) plus the checker error it produces.

match
exhaustiveness

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: Python developers who use match or if/elif chains over enums or tagged types and want missing cases caught automatically. Difficulty: Medium Python version: assert_never needs Python 3.11+ (or typing_extensions). Runtime behaviour checked on CPython 3.11; both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). Checker output is from mypy 2.3.1 and pyright 1.1.414.

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

1. Why This Even Matters — A Story First

A hotel has a checklist for every room type: single, double, suite. Each night the night manager walks the list. One day the owner adds a fourth typ

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11 min read · 11 sections

Run the code

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

PythonSilent miss, loud default, assert_never and a test that walks the enum

Practice

PythonExhaustive matching over a union of dataclasses and a Literal

Practice

Discussion

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