Mypy

• Run the type checker after every change, and treat a type error as a build failure
• Configure it under [tool.mypy] in pyproject.toml, with strict = true for new code
• On an existing codebase, enable strictness per module rather than globally weakening the settings
• Never relax a global setting to silence one error, fix the annotation or scope the exception
• Scope every ignore to its error code, # type: ignore[arg-type], with a comment saying why
• Silence third party libraries without stubs through a per module ignore_missing_imports, not globally
• Install available stub packages instead of ignoring the import
• Never delete an annotation to make the checker quiet