René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"agent: Improve test discovery for refactored callsites with different error semantics","articleBody":"## What\n\nWhen the agent refactored `_file/pyproject_toml/deps.py` to use `validated_get` (which returns a default on both missing keys AND invalid types), it changed the error semantics: the original code raised `PyprojectTOMLDepsError` on invalid types but returned a default on missing keys. The agent only ran the module's own tests (`tests/usethis/_file/`) and the wrapper tests, missing the 21 failures in downstream callers (`tests/usethis/test_deps.py`, `tests/usethis/test_init.py`).\n\n## Where\n\n- `src/usethis/_file/pyproject_toml/deps.py` — `get_project_deps()` and `get_dep_groups()` were converted from `validate_or_raise` (raises on invalid type) to `validated_get` (returns default on invalid type), silently changing behaviour.\n- `tests/usethis/test_deps.py` — 21 tests that exercise the error-raising path through `_deps.py` → `_file/pyproject_toml/deps.py`.\n- `tests/usethis/test_init.py` — additional tests that depend on correct backend dispatch through these paths.\n\n## Why it matters\n\nThe agent validated changes by running only the directly-related test files, not the transitive callers. When a refactoring changes error semantics (e.g. swallowing errors that were previously raised), the breakage manifests in downstream tests, not in the module's own tests.\n\n## Lesson\n\nWhen replacing a function that has **different behaviour for missing-key vs invalid-type** (return default vs raise error) with a wrapper that treats both the same way (`validated_get`), the agent must:\n\n1. Identify all transitive callers of the changed function (e.g. `_deps.py` calls `_file/pyproject_toml/deps.py`).\n2. Run the tests for those transitive callers, not just the direct module tests.\n3. Specifically check whether the original code distinguished between 'missing' and 'invalid' error paths before collapsing them into a single default-returning wrapper.\n\n## Suggested improvement\n\nImprove the `usethis-python-test-affected-find` skill to emphasize how to discover all potentially affected tests whenever a function's error semantics change, rather than relying on the agent's manual judgement about which test files are relevant.","author":{"url":"https://github.com/nathanjmcdougall","@type":"Person","name":"nathanjmcdougall"},"datePublished":"2026-04-04T17:33:01.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/1817/usethis-python/issues/1817"}