---
title: Verify Lint Rule Scope Before Relying on It for Enforcement
date: 2026-05-01
category: docs/solutions/best-practices
module: stemwijzer
date: "2026-05-01"
problem_type: best_practice
component: development_workflow
severity: medium
applies_when:
  - Adding a new lint rule to catch a specific pattern
  - Refactoring code to eliminate an anti-pattern
  - Writing new modules while an anti-pattern cleanup is in progress
symptoms:
  - Lint rule appears to enforce a rule but misses the variant you actually use
  - New code re-introduces the anti-pattern you are actively removing
  - False confidence that CI/pre-commit will catch regressions
root_cause: missing_tooling
resolution_type: workflow_improvement
related_components:
  - ruff
  - pre-commit
tags:
  - linting
  - ruff
  - exception-handling
  - self-review
  - workflow
---

# Verify Lint Rule Scope Before Relying on It for Enforcement

## Context

During a refactor to tighten exception handling (P2-002), I added the ruff `BLE` (blind exception) rule to `.pre-commit-config.yaml` and `pyproject.toml`, believing it would catch `except Exception:` blocks. I then assumed the rule would prevent regressions and wrote new modules (`health/`, `scheduler.py`, `scripts/health_check.py`) that contained `except Exception:` — the exact pattern the refactor was meant to eliminate.

The BLE rule **only catches bare `except:`** (no exception type at all). It does **not** flag `except Exception:` or `except Exception as e:`. The result: the new modules passed linting while re-introducing the anti-pattern.

## Guidance

When adding a lint rule to enforce a pattern:

1. **Read the rule documentation** to confirm it catches the exact variant you care about. Do not assume the rule name or description covers all forms of the anti-pattern.

2. **Test the rule against your codebase** before relying on it. Run the linter on a file with the target pattern and verify it actually flags it.

3. **Check for rule gaps.** If the rule misses a variant you use (e.g., `except Exception:` but the rule only catches bare `except:`), either:
   - Find a complementary rule that covers the gap, or
   - Add a custom rule/regex check, or
   - Rely on code review instead of automation for that variant.

4. **When cleaning up an anti-pattern, do not write new code that uses it.** This sounds obvious but is easy to violate when working across multiple files over a long session. Maintain a mental (or literal) checklist: "I am currently removing X; any new code I write must not contain X."

5. **Run the full linter on new files before committing.** Do not assume pre-commit catches everything — verify manually if the rule set is new or recently changed.

## Why This Matters

Lint rules create false confidence. If you believe a rule enforces a standard, you stop looking for violations manually. When the rule has a narrower scope than you assumed, violations slip through and accumulate. This is especially dangerous during refactors where new code is written alongside cleanup — the new code inherits the "old" bad habits because the safety net has a hole in it.

## When to Apply

- Adding any new lint rule (ruff, pylint, flake8, etc.)
- Refactoring to remove an anti-pattern across multiple files
- Setting up CI enforcement for a new code-quality rule
- Working on long-running refactor branches where new modules are created

## Examples

**Actual incident — ruff BLE rule scope mismatch:**

- Problem: `database.py` had broad `except Exception:` blocks that silently swallowed errors
- Assumption: ruff `BLE` rule would catch these and prevent regressions
- Reality: BLE only flags bare `except:` (no exception type). `except Exception:` passes silently.
- Result: New `health/checks.py`, `scheduler.py`, and `scripts/health_check.py` all contained `except Exception:` blocks. They passed pre-commit and CI.
- Fix: Remove or tighten the `except Exception:` blocks in new code. Consider adding a custom check or a stricter rule (e.g., `TRY` rules from `tryceratops` or a custom AST grep) if `except Exception:` must be banned project-wide.

## Related

- `docs/solutions/best-practices/working-tree-hygiene-dependency-groups-and-gitignore-2026-04-24.md` — related workflow hygiene guidance (pre-commit configuration, dev-tool availability)
- `docs/solutions/workflow-issues/verify-session-artifacts-against-canonical-sources-2026-04-24.md` — same principle applied to documentation: verify before trusting
