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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"agent: contextlib.contextmanager does NOT restore state on exception unless try-finally is used","articleBody":"## Lesson learned from PR #1795 (infer dependencies from setup.cfg)\n\nWhen using `@contextmanager` to implement `UsethisConfig.set()`, the cleanup code after `yield` does NOT run if an exception is raised inside the `with` block — UNLESS the code uses `try-finally`.\n\n## Root cause\n\nPython's `@contextmanager` works by re-raising exceptions at the `yield` point inside the generator. Without a `try-finally` wrapper around `yield`, the code after `yield` never executes when an exception occurs:\n\n```python\n@contextmanager\ndef set(self, *, quiet=None, ...):\n    old_quiet = self.quiet\n    self.quiet = quiet\n    yield  # If exception raised here → code below NEVER runs!\n    self.quiet = old_quiet  # ← NEVER runs on exception without try-finally\n```\n\nThe fix is:\n```python\n@contextmanager\ndef set(self, *, quiet=None, ...):\n    old_quiet = self.quiet\n    self.quiet = quiet\n    try:\n        yield\n    finally:\n        self.quiet = old_quiet  # ← ALWAYS runs, even on exception\n```\n\n## How this caused cascading CI failures\n\n1. My changes made `get_dep_groups()` require `SetupCFGManager()` to be locked\n2. Tests without `SetupCFGManager()` in their context raised `UnexpectedSetupCFGIOError`\n3. This exception propagated through `with usethis_config.set(quiet=True):` blocks in production code (`_core/tool.py`)\n4. Without `try-finally`, `quiet=True` was NOT restored after the exception\n5. All subsequent tests produced empty output (since `quiet=True` suppresses all print functions)\n\nThis caused ~80 cascading test failures across `test_console.py`, `test_base.py`, and all `_tool/impl/base/` test files.\n\n## Fix applied in PR #1795\n\nAdded `try-finally` to `UsethisConfig.set()` in `src/usethis/_config.py` (commit bfb2905).\n\n## Recommendation\n\nAny `@contextmanager` that modifies shared state MUST wrap the `yield` in `try-finally` to guarantee cleanup runs on exceptions:\n\n```python\n@contextmanager\ndef my_context_manager():\n    # setup...\n    try:\n        yield\n    finally:\n        # cleanup (always runs, even on exception)\n```","author":{"url":"https://github.com/nathanjmcdougall","@type":"Person","name":"nathanjmcdougall"},"datePublished":"2026-04-04T18:11:28.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/1818/usethis-python/issues/1818"}