Add usethis-python-enum skill and dict comprehensiveness test for _STATUS_TO_CLASSIFIER_MAP

Copilot · nathanjmcdougall · Copilot · commit 2b5f0231e66c · 2026-03-25T00:24:57.000Z
Co-authored-by: nathanjmcdougall <18602289+nathanjmcdougall@users.noreply.github.com> Agent-Logs-Url: https://github.com/usethis-python/usethis-python/sessions/8b73af53-8aa9-4d64-903e-93112ce1ac5d
diff --git a/.agents/skills/usethis-python-enum/SKILL.md b/.agents/skills/usethis-python-enum/SKILL.md
@@ -0,0 +1,47 @@
+---
+name: usethis-python-enum
+description: Style and testing conventions for working with Python enums
+compatibility: usethis, Python, pytest, enums
+license: MIT
+metadata:
+  version: "1.0"
+---
+
+# Working with Enums
+
+## Exhaustiveness via `assert_never`
+
+When handling all cases of an enum in `if`/`elif` chains, always include a final `else` branch that calls `assert_never` from `typing_extensions`. This ensures the type checker will flag any unhandled enum members if the enum is extended in the future.
+
+```python
+from typing_extensions import assert_never
+
+from usethis._types.backend import BackendEnum
+
+def handle_backend(backend: BackendEnum) -> str:
+    if backend is BackendEnum.uv:
+        return "uv"
+    elif backend is BackendEnum.none:
+        return "none"
+    else:
+        assert_never(backend)
+```
+
+## Dicts with enum keys must have comprehensiveness tests
+
+When a module-level dict uses an enum's members as its keys, add a unit test that asserts the dict's keys exactly match the set of all enum members. This prevents the dict from silently becoming incomplete when new members are added to the enum.
+
+### Example test
+
+```python
+from usethis._core.status import _STATUS_TO_CLASSIFIER_MAP
+from usethis._types.status import DevelopmentStatusEnum
+
+class TestStatusToClassifierMap:
+    def test_keys_match_enum(self):
+        assert set(_STATUS_TO_CLASSIFIER_MAP.keys()) == set(DevelopmentStatusEnum)
+```
+
+### Why this matters
+
+Type checkers can catch missing cases in `if`/`elif` chains when `assert_never` is used, but they **cannot** catch missing keys in a dict literal. A dedicated unit test is the only reliable way to ensure that all enum members are represented.
diff --git a/tests/usethis/_core/test_status.py b/tests/usethis/_core/test_status.py
@@ -3,12 +3,17 @@
 import pytest
 from pydantic import TypeAdapter
 
-from usethis._core.status import use_development_status
+from usethis._core.status import _STATUS_TO_CLASSIFIER_MAP, use_development_status
 from usethis._file.pyproject_toml.io_ import PyprojectTOMLManager
 from usethis._test import change_cwd
 from usethis._types.status import DevelopmentStatusEnum
 
 
+class TestStatusToClassifierMap:
+    def test_keys_match_enum(self):
+        assert set(_STATUS_TO_CLASSIFIER_MAP.keys()) == set(DevelopmentStatusEnum)
+
+
 class TestUseDevelopmentStatus:
     def test_planning(self, uv_init_dir: Path, capfd: pytest.CaptureFixture[str]):
         # Arrange
