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Add B047: except handler shadowed by an earlier one - #589
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cooperlees merged 3 commits intoOct 9, 2026
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A handler like `except ValueError:` placed after `except Exception:` can never run, since the earlier handler already catches every ValueError. The same goes for a bare `except:` after `except BaseException:`. Like B014, only builtin exceptions are checked, since we can't know the hierarchy of other classes from the AST. Catching the exact same name twice is still left to B025. Closes PyCQA#349
…cept-handlers # Conflicts: # README.rst # bugbear.py
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🟡 Changes recommended
B047 misses unreachable builtin exceptions inside valid starred handler tuples.
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What changed in this PR
This PR adds B047 to flake8-bugbear to flag except and except* handlers shadowed by earlier handlers for builtin exceptions.
Changes:
- Add the B047 check and diagnostic message.
- Add examples and regression tests for ordinary and starred exception handlers.
- Document B047 in the README and unreleased changelog.
| File | Description |
|---|---|
| tests/eval_files/b047.py | Tests ordinary handler ordering. |
| tests/eval_files/b047_py311.py | Tests except* ordering. |
| README.rst | Documents B047 and its release note. |
| bugbear.py | Implements and registers B047. |
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| elif isinstance(handler.type, ast.Tuple): | ||
| names = [e.id for e in handler.type.elts if isinstance(e, ast.Name)] |
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Good catch, B047 now collects names through _flatten_excepthandler, so starred tuple entries are expanded, and the except (*(ValueError,),): case is covered in b047.py.
cooperlees
approved these changes
Oct 9, 2026
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B047 reports an
excepthandler that can never run because an earlier handler in the sametryalready catches a parent class:It also covers
except BaseException:followed byexcept Exception:or a bareexcept:, which was the first case in the issue. Works forexcept*too, and for names inside tuples.Like B014, it only looks at builtin exceptions since we can't know the hierarchy of other classes from the AST. Catching the exact same name twice is still left to B025.
B047 is the next free code after B045 and B046. Happy to renumber if needed.
Closes #349
Tested with
pytest tests/test_bugbear.py(3.10, 3.12, 3.13, 3.14),pre-commit run --all-files, and running it over the stdlib and site-packages with--select B047, which found nothing.